The motherboard is a very, very important equation in this, which a lot of people overlook. If you're going for AMD64 (my personal next upgrade option) there are socket 754 versions which are cheaper, but the socket 939 range is the most future-proof. I work for a university hardware department, we've used loads of different manufacturers over the years, and discovered that Asus (Asustek) motherboards seem to be te most reliable, and the best to depend on. I know Abit boards come highly recommended by other people on this board too, so that's an option.
RAM: Get Dual Channel. Spend a bit extra getting some proper brand-name RAM. Beware of some PC vendors selling "major brand" RAM. There's a shop near us that lists Major Brand RAM. We bought some once and it was by some piece-of-crap company that we'd never heard of. We generally use Hynix or Samsung RAM, but there are plenty of other decent RAM manufacturers, Cruical (Micron), and if you're planning on overclocking, Corsair, OCZ and Mushkin are generally well received (though i've never had any personal experience with the last two). There are other types too- best thing to do is read loads of reviews online.
Hard Disc - there's no reason to go for Parallel ATA any more - serial ATA is the way to. A couple of companies are offering Hard Discs with 16MB cache. One thing to say: there is no such thing as too much hard drive space. I've got 240GB in total on my PC and i'm having to watch what I use
There are other things to consider, video cards, monitors etc, but I need to go out, so someone else can carry that on :-P
Marc - maybe me and some of the other PC geeks on here should write a regualr column on "recommended video editing systems" on a couple of different budgets?