Sadly, with that computer, you are severly limited. Higher performance cards consume alot more power and benefit most when paired with a higher performance processor/motherboards. Depending on your circumstances, you might be best served by looking for a new/refurbished desktop from a reputable supplier (warranty reasons). Microcenter, Tiger Direct, Fry's etc.
Intel I7 960 @ 3.87ghz *Intel DX58SO *HIS HD6970 2gb *Corsair TX650M *2x4gb Corsair XMS3 *WD Black 1TB *Windows 7 Home Premium 64bit *Gateway HD2401 24" 2ms 1920x1200 ***Asus N71Jq laptop *Intel I7 720QM Processor *Mobility Radeon HD5730 1gb *8gb Ram *Windows 7 64bit
* A clear conscience is usually a sign of bad memory *