As a part-time computer builder/gamer/tech nut the single most important piece of computer hardware is your power supply. Without a good power supply your computer will NOT run well and you're more apt to see BSOD's then someone with damaged hardware. The ONLY power supplies I will recommend you use are PC Power & Cooling (simply the best out there, Enermax, Adapted and most recently BFG Tech. I only used to use Adaptec supplies in my own machines for their reliability, price and power but I've recently built a new rig around the BFG Tech 650w and it runs supper quite with stable output. If you do a test on your power supply, most of you will find considerable differences between what it should be, and what it is putting out ESPECIALLY if you have a stock power supply from the manufacturer.
The second most important part of a PC is memory. YOU CAN NEVER HAVE TOO MUCH (as long as you're running Windows XP SP2 or higher). Windows 9x kernal doesn't play well over 128MB. I currently have 2GB of PC2-6400 @ 800Mhz and I find this is the minimum amount of RAM for any true game rig. If you're planning on playing BF2, 2142, Supreme Commander or just about any game released in 2007 then this should be your starting point, even before CPU speed. I'm planning on upgrading to 4GB as soon as I can.
Third on the list of important things to have in your computer if you're going to play high-end games is a great graphics card. Again, like the memory, you can't have too much. I had planned on putting an BFG (again, the ONLY card I will use in my machines PERIOD) 8800 GTS OC 640mb PCIx card in this machine, but opted for the 7950 GT OC 256mb for price at this time. The BFG 8800 GTX OC card is the fastest single video card on the planet at this time, so if money is no object, this IS the card you will want. The biggest difference between the 7900 and lower cards to the 8800 cards, is that the new 8800 cards have Physics chips built in, so you wont need another add on card to get the benefiet of this awesome effect. Many people complain about the price of BFG cards when I recommend them, but all I can say is you get what you pay for. They are one of the few video card manufacturers that stand behind thier product with a FULL LIFETIME warrenty, and they even sell pre-overclocked cards with the same coverage.
Number four on our list of this to put in your rig is the CPU. Some people question why it's not number one and that's because CPUs are outrunning programs for once. It used to be you had to upgrade to a new CPU as soon as they came out so you could run another TSR on your slow machine. Now with CPUs hitting the 5 Ghz mark along with dual and quad cores, it should be a while before you HAVE to upgrade. (Although Vista should make things interesting). As long as you're running something in the 3Ghz or higher you 'should' be ok to game as long as you follow the above items. My last game rig was an 820 D dual 2.8 machine with 2GB and a BFG 7900 and it ran BF2 with everything as high as it would go at 1600x1200@32.
To the fifth and final requirement to a good game rig is hard drive. Once again, too much is never enough, but speed rules here. I ditch my CD/DVDs as soon as the game is installed and run from a mini image, so all the game info is comming directly from the drive. My current system boots from a 36GB Raptor drive on SATA1 and my games/apps are installed to a 500GB SATA2 drive that screams while I'm playing. Always look for drives based on 7200 rpm or faster and Serial and RAID can be your friend.
Well, I hope this helps some of you.. I know it works for me..
Intel DG965WH motherboard (DO NOT BUY THIS BOARD)
Intel E6300 @ stock 1.8 Dual Core
BFG Tech 7950 GT OC 256MB
Onboard hard hi-def sound
Onboard Gigabit LAN
Boot - 36Gb Raptor
Install - 500Gb SATA2
Saitek II Keyboard
Razer Copperhead mouse
Envision 19" CRT (fast and big) ~ LCDs are for laptops, and laptops aren't for games