Building your own PC is not only cheaper than buying one pre-made, but it allows you much more customization.
You may be replacing and older system, or perhaps just want to upgrade your current one. As complex as computers are, the assembly of their parts is far easier than most people think. Most everything has a slot, plug, or connector that pops right into place. You'll only need a screwdriver, and you're set. Just be sure to keep yourself grounded by touching a moderately sized metal object before handling the PC components themselves. Always carry circuit boards, hard drives, and processors/ram by their edges (not the edges with connectors on them obviously).
First of all, have a clear plan of what kind of system/upgrades you're looking for by prioritizing what applications you'll mostly be using. If you're just going to be browsing the web, running spreadsheet programs, watching DVD movies and playing mp3s, you won't need too powerful a system. That's not to say you wouldn't benefit from having a top of the line system, you just would not benefit from it as much as if you were heavily into cad/video/3d editing, or playing the latest games.
Think about if you really need an entire new system, or if just a few parts. More ram is probably one of the upgrades I hear about the most, and as much at it does help you run more programs at once, or have less studder with resource hungry games, it doesn't really make your system that much "faster" per se. A good cpu will help with the general speed of your system, but 3D game performance/quality is heavily dependant on the video card. If you're looking for huge performance boost with minimal cost, you should consider upgrading your CPU/Motherboard/Ram/Video, and keeping the rest of your system components if they're still functioning and adequate. Your drives and cards as well as your peripherals should work fine on the new motherboard, saving you a lot of cash from buying newer ones that you may honestly not even need.
Basic Computer Parts
The main components of your "computer" itself, excluding things like monitors, keyboards and mice, crap like that. Just the computer box itself, and the stuff inside it.
You can consider this the "core" of the PC, everything attaches/plugs into the motherboard or something attached to the motherboard. Your motherboard dictates what can be plugged into it and is therefor one of the major deciders of what hardware your sytem will be able to support. It's best to select which processor you want first, so you can narrow your search to just the motherboards that support that chip. Some chips come in multiple sockets, so you'll have to decide whether to have a motherboard that will only support that one chip and not much faster, or one that will support more advanced chips/technologies such as dual/quad core or above. Some of the major deciding factors on choosing a motherboard are: what CPUs does it support? How much ram will it support, and which speeds? Does it have PCI, AGP, or PCIE for video? Does it support SLI? How many IDE/SATA slots? Does the bios support overclocking features? Does it have on-board audio/video/raid/wireless?
The most important part of the PC, the "brain" if you will, is the processor. The speed and complexity of the CPU determines how well you run applications and games, how many you can run at once, or in some cases if you can even run them at all. Dual core processors are able to function as if your system had two physical CPUs, and would be able to benefit applications that take advantage of multiprocessors or hyperthreading. While dual core CPUs are no-doubt faster than a lot of their single-core counterparts, not all programs and applications are able to fully utilize both processors, and may only work on a single core at once. Although getting a faster CPU will certainly speed up your system, if your video card is not up to par, you may not see that much an improvement in 3d games. On the other hand, if you have a high grade video card but a very low power CPU, then the CPU will be the bottleneck. There isn't a specific method to find out whether your CPU or video card is the bottleneck, but looking for benchmarks of your video card alongside different CPU configurations, and then your CPU with other video card configurations should give you a good idea of where you stand. I can' tell you for sure that as far as the latest, heavily graphical games, you'll need at least a dual core processor, and a non-onboard videocard with at least 256mb of vram, preferrably a geforce 7/radeon x1750 or faster. Most dual core processors are also able to run 720p HD video, however 1080i and 1080p may need a bit more power. Some newer video cards help offload rendering from the CPU, but they only reduct CPU usage, so the ability to run HD video is still mainly dependant on the CPU. Your CPU choice will determine what motherboard you get, so best to choose which processor you want to narrow down your choice of motherboards.
Highest grade: Intel Core i7, AMD Phenom
Medium Grade: Intel Core 2 Duo, AMD Athlon 64 X2
Low Grade: Intel Pentium 4, AMD Athlon
Legacy: Intel Pentium III, AMD K6-III
LGA 1366 = Core i7
LGA 771 = Core 2 Extreme QX9775
LGA 775 = Core 2 Duo E6300 - Core 2 Extreme QX9650
Socket 478 = Pentium 4, Celeron, Pentium 4 Extreme Edition
Socket 423 = Pentium 4, Celeron
AM2+ = Athlon 64, Athlon 64 X2, Athlon 64 FX, Opteron, Phenom
AM2 = Athlon 64, Athlon 64 X2, Athlon 64 FX, Opteron, Phenom
Socket 939 = Athlon 64, Athlon 64 FX to 1 GHz, Athlon 64 X2 to 4800+, Opteron 100
Socket 754 = Athlon 64, Sempron, Turion 64
Since you usually choose the motherboard and processor first, you then select the ram based on what the motherboard is compatible with. The specifications in the manual or online at the manufacturer's site should show you what range of speeds and what size(s) supported. RAM will have different types like DRAM SDRAM DDR DDR2 and DDR3, and speeds like PC133 PC4000 PC2-6400 and the like. Some motherboards support a technology called "dual channel" for the memory in which you can use pairs of them for improved performance. Nowhere near double, but enough for the higher end crowd to almost need it as a requirement. A good size for an economy PC meant for mostly desktop applications will run well with any OS outside of vista with 1gb of ram, a midrange system that would expect to run some of the latest games, or some more demanding programs, would be better off with 2gb. With vista, add 1gb to those numbers. 4gb and above is frankly too much for the average game player or desktop worker, with the exception of some memory intensive applications like online servers or graphics programs, you won't see any improvement. Also, if you are still using a 32 bit operating system, you may not be able to use any ram above 3gb. This is due to a limit of the amount of memory that can be referenced in 32-bit. In order to be able to access more, you would have to use a 64 bit operating system. It should be noted that 3gb is enough for most games, and any more will not have a noticeable impact on performance, and is therefore not worth changing your OS over if you are comfortable with what your currently have.
Current Ram Speeds:
High-grade: DDR3 1600
Mid-grade: DDR2 800
Low-grade: DDR 400
Legacy: SDR 133
First, let me get something straight. There is no correlation between the amount of ram on a video card and how fast the card is. The memory on the video card is there to give the GPU a fast resource for memory, having more of it would only help with bottleneck situations where you're filling the ram with too much data either because of too high of graphics settings or too high of resolutions. For games several years old 128mb will do the job. For more recent games, up to just a few years ago, and even some of the newer ones, id suggest at least 256mb. And for the absolute newest games, or for running in maximum detail with AF and AA, you'd be best off with 512mb.
This is the current format, and the fastest available. There is some confusion over the speed of PCIE in relation to your video card. As I and countless benchmarks on the web can tell you, there is no perceivable difference between 8x and 14x on most video cards. PCIE 2.0 is the newest version of PCIE, and it allows for even HIGHER rates than 14x. Since we haven't even been utilizing 14x yet, not having PCIE 2.0 is nothing to worry about. By the time video cards come out that need more than 14x, you will no doubt be wanting a better processor since it will assuredly be bottlenecking it. PCIE also brings a new technology to the table SLI and crossfire. By using a motherboard with two PCIE 8x slots, that supports either technology (and it's usually one, and not the other), you will be able to pair up video cards to increase your rendering power. The cards must be SLI (nvidia) or Crossfire (amd) capable, and you'll have to connect a sli "bridge" between the cards, and turn on a switch/jumper/bios setting on your mobo (dependent on your mobo, some even do it automatically). There are some up-sides and down-sides to doing this. The down-side would be that you're not getting exactly 100% improvement in most instances, but still paying 100% for another video card. You increase the load on your power supply (it's a good idea to use SLI or Crossfire certified PSUs, but to be safest, check nvidia.com or ati.com and see what wattage and rail amperage they reccomend for your card combination). Further increasing your GPU power will put extra strain on your processor, so if it isn't fast enough to supply both your video cards, it could be holding back the gain you would get from two cards. The up-side is for only the price you paid for your first card (minus declining prie), you get a noticable performance boost when playing newer games, or using maximum video settings and resolutions. Games not benefitting from multiple video cards could still be improved by letting the other card(s) do AntiAliasing. There's of course tri- and quad-sli for the very-absolute-highest framerate out there, but the scaling after two cards is currently pretty meager, and honestly not worth another full payment for a video card in most instances. Something else to keep in mind while thinking about SLI or Crossfire, if you're on a 32 bit operating system, your maximum available ram will be reduced by the amount on your video cards. For instance a system with 4gb of ram and two video cards with 1gb of ram each, will let you only use 2gb of your system ram.
These standards are outdated, and you will not see much improvement in video performance even with the the best cards for either slot, at that point you would benefit more from a CPU/Motherboard upgrade, and just getting an entry-level PCIExpress card. If you MUST get a new video card, the geforce 7 series or one of the 2xxx radeon series would be the fastest that you'll find.
Current Video Cards:
High-grade: Nvidia GTX 280, ATI Radeon HD 4870 X2
Mid-grade: Nvidia Geforce 8800 GT, ATI Radeon HD 3870
Low-grade: Nvidia Geforce 7600 GT, ATI Radeon X1900 Pro
Legacy: Nvidia Geforce FX, ATI Radeon 9800, most on-board video
Advantages: best color quality, black/white levels, resolution range, fastest refresh rate.
Disadvantages: Big, heavy, high energy usage, high heat output, expensive to repair, mostly in 4:3 ratio.
Advantages: small, light, low energy, low heat producing, cheap to repair, mostly in widescreen ratios.
Disadvantages: black levels, displays non-native resolutions blurrier or smaller, old models have problems with viewing angle or "ghosting"/"tracers" when showing movement.
To come later.
Input devices range from touch pad to touch screen to the stupid eraser head in the middle of laptop keyboards, but mice are generally considered the best for most games and non-graphical applications. Ball mice are inaccurate, clumsy, and get dirty fast which requires frequent cleaning. With optical mice being so affordable nowadays, you have no excuse not to increase your comfort and probably even your productivity by getting one. From a gaming standpoint, you'll prefer a wired, USB mouse for the best sampling rate. Wireless and P/S2 mice don't poll for mice position as much as a wired USB mouse, so mouse movement in 3d games will appear more jerky and less responsive as a result. Wireless mice also need to have their batteries changed, and they are vulnerable to interference, and can have sudden disconnects and need to be re-synched. Most mice nowadays have a scrollwheel/middlemouse button, however some other mice boast features such as additional buttons, a 2 axis scroll wheel, or even increased resolution of movement sampling.
Rather than write here what has already been done so comprehensively, I leave you with some links to sites with good explanations and diagrams of each part of the procedure. I highly reccomend the first link for most in-depth information, although some may be more comfortable with the other sites that use less techno-jargon and more visuals.
I apologize for the fragmented nature of this post, but it is under constuction and will be continually updated by me, GoldenRatio, and JohnnyCasil. Any suggestions for improvement from other users is greatly appreciated.