Heya,
RAM is a big one; and it's best to use Vista64 where you can get all the RAM you can handle (8 gigs should be a good place, and it's dirt cheap to get that right now) and Vista64's handling of memory--far superior to XP and 32bit versions of Vista. So soak up some yummy RAM and use it in a proper OS, Vista64.
CPU is important, but it's not like you have to go out and buy the latest $1,000 extreme Intel edition thing. You'd do completely fine with an Intel Q6600 for example, or a Phenom 9850 BE. Both are highly overclockable (if you like) and are in the $150~180 ranges. Very affordable for some serious performance (and both can be clocked up to 3ghz on just air cooling, ie, a good fan/heatsink with minimal effort). Quads do make a difference compared to Dualcores by the way; so go Quad. If you can afford it, the new i7 core Intel material is very fast, as well as the memory; the total throughput of the new i7 core stuff is just quite a bit above our current best Quads. Entry level is $300+ though, and the memory is pricey too. You can easily spend double on an i7core build. But it will be a very ferocious machine.
Often overlooked is the HDD speed. A single 7200rpm just won't do likely. For one, it won't keep up with the massive size of videos as you edit. For seconds, they typically just run out of space unless you get the latest 1tb and 1.5tb flavors, even then, you still easily run out of space. You could invest in expensive faster drives (10k rpm, SCSI 15k, etc) but that's just going to up the cost without any real benefit when you could just go with RAID and inexpensive disks. That said, the WD black caviar 1 terabyte drives are wonderful. Three of those in RAID0 will give you a monstrous 3tb drive to work on with speed that will keep up with your output. The ideal thing to do is to have your OS/applications on a separate drive (you could use an SSD for this, for really good speeds) and have your actual files that you write/save on that huge fast RAID array. That way, things are separate. If you feel the need to backup stuff, a Bluray writer could be a good option for you.
Anyhow, to say, it's not just one component that will make your system really work out for the video editing, but a combination of the above. Memory, CPU and HDD speeds matter. Storage capacity matters. And of course, power reliability.
Very best,