Some good advice on this site, but i would suggest doing some research yourself. It honestly is by far the best way.
Goto www.tomshardware.com and check out these two links.
http://www.tomshardware.com/reviews/gaming-graphics-card-geforce-gtx-480,2598.html
http://www.tomshardware.com/reviews/best-gaming-cpu,2599.html
Also go to the forums on tomshardware, there is a whole community of people that will help you with your build.
Your computer as others have stated will most likely not run pcie cards and you cannot upgrade to an am3. I just recently built a pc for under $500 and i live in australia so our prices are alittle higher. If you can afford to spend alteast $500 you will be able to get a decent gaming rig but if you want to play crysis, and play it well i would aim closer to $700 not including monitor.







