Your processor is going to be your bottleneck, I'm assuming its a Core2Duo. No need to spend a lot of money, because that processor isn't going to be that great at 3D games. Check out the Best PCIe Card for the Money April 2011 article
, then look for deals on Newegg.
Don't let the fanboys fool you, both ATI and Nvidia make good cards. If you choose one of the suggested cards in the TH article, you can't go wrong.