$400 is a real stretch if you're looking to build even the most basic of gaming rigs. I spent $900 on mine(noting that if I hadn't had friend build it for me, I would have ended up paying nearly 2 grand for it). Your best bet would be to save up another few hundred and then get a custom build. I'm not sure of computer prices in your area though, so you may have more options than you would up here.
Edited because spelling and grammer pwn me
Edited, Mar 9th 2009 7:55am by Driftwood