Ok, I'd go for a 64bit Athlon, or a P4 Dual-core. You might be able to pick up an older P4 (without dual-core, but has a high clock speed). I'd get at least 1gb of ram. Don't skimp on a video card. This machine would be for editing right?
I just won an athlon 64 x2 3800+ in a film contest .... wanna buy it? (i have a mac and you would have to wait 4 more weeks for it to come.) No ... ok then.
I would suggest building your own as well, my personal favorite site is tigerdirect.com, they have a lot of good barebones kits. My roomie just got a new laptop from ibuypower.com and it kicks ass (was like $1500). I would check there first.