check these out, $501 on newegg. just replaces everything that needs to be replaced for game speed reasons. you can keep your hdd, and optical drives. Go to newegg.com and put in these item numbers:

N82E16811156098
N82E16814150316
N82E16820231084
N82E16813131347
N82E16819115037

should build you a pretty decent machine, and you'll be gaurenteed everypart is compatible but upgradable. The only thing you wont be able to do game related is SLI or Crossfire setups.

EDIT: forgot one. Added.