I have about $400 left for the GPU. The goal is to game. I don't know what GPU to get, or if I should get two, or perhaps even swap out some other parts. Some games I want to play are: SC2, Civ V and America's Army, but I also want to be able play games that will come out in the future.
Edit: My resolution is 1280x1024 for now.
Edit2: Updated my PSU, thanks Embra for the great find!
More aboutanother what thread
If you can find one for around $400 get a gtx 580. Definitely the best single card performance. Otherwise consider crossfiring two 6870's. The latter is cheaper but Xfire can have problems
Have you bought your PSU yet? and what resolution are you using?