How long do you plan on having the computer dictates the parts you will be needing.
If you change computers every year or so then upgradeability doesn't really matter.
If you plan on getting a computer that you plan on keeping for two years or more then you need to pick parts for future proofing your system.
If you cannot get the latest and bestest parts you can get away with a motherboard that will allow you to do upgrades to keep your system current. For example, various motherboards that use dual core processors will allow you to upgrade to a quad core one at a later time.
Some motherboards take RAM of various speeds and sizes so you can also upgrade to faster or bigger RAM modules at a later time. The most important part to research first for a computer is the motherboard as this is the component that will dictate the future of your system, not to mention how it will run as a whole.
Then comes the processor, try staying in the middle of the road if you are not looking for maximum power. The speed difference between two processors could be just of 20% but the price difference could be between $100-$200.
Pick a processor that is compatible with the motherboard's socket, you can't just fit any processor on a motherboard. I don't need to say that a socket 478 processor won't fit on a 775 socket (same goes for AMD equivalents).
For working better on heavy graphics get a dedicated video card, stay away from the motherboard integrated solutions. For PCI-Express cards the most basic ones range from $120-$150, these usually feed off from the PCI-Express port.
The mid-range graphics cards go for about $150-$240, the most powerful ones go from $250-$1000. Keep in mind that these more powerful cards usually require being connected to the power supply.
Next, comes the power supply. For a basic system a 300w-400w power supply is more than enough, for more powerful systems, especially those holding multiple hard drives, optical units or a power hungry video card(s) you need a power supply between 600w-1000w.
A word of advise: DON'T cheap out on the power supply, it is one of the most important parts of the computer as it regulates the power coming inside it. Cheap power supplies tend to have lower quality components and if they fail they cannot only kill themselves but take the whole system along!!!
Those $20 something no-name power supplies can look like a good deal but they're not, for power supplies get better ones, especially from a known brand, they are more expensive but they have better components and protection against spikes, not to mention they work better under heavy workload. Good power supplies start ranging from $60 and up.
Hard drive: Depends again on what do you need to do. If you are doing something like basic computing or programming then a single hard drive will do. For more heavy work, two (or more) hard drives are recommended. Some applications like video editing or those that render demand all the attention they can get from the hard drive. If both the system and the application are using the same resources from the hard drive both could "fight" over these resources and reduce performance, cause stuttering issues or cause capture/publishing errors.
For work, 7,200+ RPM hard drives are the most recommended choice. Besides, if the system hard drive fails or Windows crashes all your work will be safe on the work hard drive. And of course, the golden rule for storage devices is backup, backup, backup!!!!
Next comes RAM, the minimum amount of RAM now is now 1GB, optimal being 2GB+. Again, RAM depends on the amount or type of applications you are working on or how many you have open.
Newer systems use DDR types of RAM, to obtain the speed DDR RAM provides you need to install the memory in pairs. Do NOT combine memory speeds or capacities, while your computer may seem to work with combined memory types this could lead to issues and the computer will not run at the speed it is supposed to run.
If you use a faster RAM module along with a slower module, the faster module will clock down to match the speed of the slower module, thus neglecting the speed gains you would get from the faster module. Same happens with computers with multiple video cards, get cards that are matched in speed.
Also make sure to investigate if the motherboard uses DDR, DDR2 or higher RAM, and what speeds it accepts. Some motherboards can work with only 667Mhz RAM while others take RAM of speeds from 667Mhz to 1Ghz and higher.
Processor:
Choosing a processor depends on what you need to do. The faster the processor the faster you can get things compiled, rendered or handled.
There are 32-bit processors usually the older Pentium 4 based processors that are still powerful, especially the last ones to come out. The newer dual core processors are now 64-bit processors, this proposes some future proofing as future 64-bit applications will take full advantage of these.
Various applications do run better on 64-bit processors, some can even detect them on a 32-bit Operating System. Dual core processors are faster indeed (don't let the lower clock speeds fool you), and professional applications benefit largely from those. While running at lower clock speeds, dual core processors can process more data per clock cycle. Not to mention that a dual core processor is better at multitasking.
You can get away fine with most stuff with a 2.0Ghz dual core processor. One thing you have to be clear of and which is a common misconception is that dual cores do
NOT double the clock speed of a processor. A 2.0Ghz 64-bit dual core processor for example is
NOT a 4.0Ghz 128-bit processor, in this case you simply have two 2.0Ghz 64-bit cores running at the same time and
both cores are sharing the workload.
Optical units:
Computer Blu-Ray, HD-DVD, DVD-RW or CD-RW units are NOT only for burning music or movies.
They can also backup large quantities of data either for archival purposes or presenting work at some other place. Pick one that is a known brand, also, don't cheap out on the discs either.
You will find no name branded discs really cheap, these are fine for handing over work or files to someone else or storing non-critical data. These, however, tend to have readability problems a few months later.
If the data you are backing up is critical or you may need it later always pick media from a known brand, keep it in their box and DON'T handle the disc unless you really need it. Keep them away from moist places or those that are prone to fungus.
Next a media card reader, if you are working in graphics you need one of these definitely, not only because they can read the storage cards from cameras but you can also use the cards as storage devices for storing and carrying regular data as well.
A media card reader with a high capacity card reader can currently store up to 8GB+ per card on those new HDSC cards. If you want to add a floppy disk unit that is up to you, it's probable you won't need it in this time and day but when you need it, you need it.
There are some floppy disk/card reader combo units that offer both in the same unit.
Fans: These will ensure proper cooling of a system, a regular system will run fine on a single fan. More powerful systems have at least two or four, if fan noise is an issue pick ball-bearing silent fans. You can also use water cooling for more silent operation or if you are looking to overclock the system but these could be somewhat complicated to work with and may require periodic maintenance so these are not definitely for the average Joe.
However, fan noise is not an issue if you are running 500w speakers on your system.
CS:S admin. PM DennyCrane for assistance. 
