something i'd like to point out. if you're just looking to run stuff like that (which, if he's talking PS2 emulators, that's a way different problem with the graphics stuff), i'd suggest going with the e6750 processor. it's a 2.66ghz processor, huge fsb, and it's 190$ at newegg. beyond that, you could probably piece something together for cheap without too much effort - a mid tower case (the centurion 5 by cooler master is great), cheap mobo (gigabyte makes good low-end atx and microatx mobos), at least 2 gigs of ram (~40$), and a decent power supply that'll have overhead should you choose to upgrade. if you're running more than emulators, look into one of the 8600 cards - they've got a decent amount of power for about 100$ after rebates. you'll want a graphics card, but you might even be able to go as low as the 8500 or 7600 cards and still be able to play modern games as well.
stay xp. vista sucks balls.
if you want, just message me via pm or AIM and we can talk a bit about what you want to do and how much it'd cost to do it. i'm really cheap if you want me to build it, too.