http://contraildesigns.com/?p=310
That's a part picking guide I wrote, for yoru budget range. Basic overview of your computer:
MSI P43 MOBO
E7400
4GB RAM
640GB HDD
9800GT(COD4 included!! Or pick a slightly different one, 5 bucks for, but comes with Cod5)
DVD Burner
Awesome case
530W PSU
operating system
you said you dont need an OS, so instead of the E7400, you could get a better quality PSU. Of course, this also means that you can shoose the same case, just get the cheapest one of that kind, and you saved yet more money. Also, instead of the E7400, you could get the E8400 or the Q6600. Hope it helped!