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!