You've got a few options, you can by a ready-to-go system which is preconfigured with everything you need (most expensive). Or you can modify an off the shelf system by adding a video capture card (like the Matrox RT2500 or Pinnicle) and perhaps some big fast drives. Or you can build a system from the ground up. The last option is the least expensive, but adds the headache of installations, configurations and troubleshooting. Plus as you are finding out now, figuring out what components to get.
I recommend starting out by defining your goal. I am building a system to feed in video from multiple sources (miniDV and SVHS), editing using Adobe Premiere, After Effects, Illustrator and Photoshop, sound editing with another program (yet to be determined) and outputing as MPeg2 to a DVD-RW and/or video tape and/or streaming video.
I start with determining the motherboard and any special add-in cards (like the RT2500) then choose a processor compatable with the board and memory (remember you can buy a processor a step or two below the cutting edge (most expensive) and upgrade later). Same with memory, 512mb now is probably fine, go up to 2Gb when the prices fall and you are up and running at a profit.
Next you'll need to look at hard drives which should be at the very least big 7200rpm ATAs and ideally a RAID of 10,000rpm Cheetahs or similar. You will need a suitable video card, monitor, CD drive, floppie, keybord, etc. And of course, don't forget to select a case which will provide adequate space, power and cooling.
Then all you need is a ton of softwaer and enough material to edit something worth all of this trouble. Good Luck!
John