Newegg has a bunch of combo deals. For 300 bucks you can have a motherboard with onboard audio, proc, video card and maybe even some memory. A cheapass case with the cheapest antec power supply you can get your hands on and you're set. Maybe a new hard drive. Computers are cheap, dude.
Built mine for about $800. Overclocked to 2.8GHz on the stock HSF too.