First let me ask some questions:
[strike]1. You're using PCI-Express 16x I'm assuming?
2. What is the wattage/amperage on your power supply unit?[/strike]
3. What resolution are you gaming at?
Assuming that you are using PCI-E, have enough power to run any card, and are playing at a resolution of 1280x1024, I recommend a NVIDIA 9800GT or an ATI HD4830... and that's the maximum.
Here are some factors. If you're playing at a low resolution, such as 1024x768, then you will get severe bottlenecking, and taking the resolution up to 1280x1024 will not make much difference. If you are playing at a higher resolution, such as 1600x1200, then bottlenecking will not be as much of an issue (though certainly still present).
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127381
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102803