To be blunt here, no high-end graphics card will work in your system. You power supply will not support it, and the rest will just be a bottleneck. You're looking for something in the low end of the midrange unless you just want to waste money. The 5670 recommended above or something like the GTS250 would work fine but you won't be playing anything maxed out.
Still not enough information to make a good answer. Your CPU speed (GHz) is not as important as the series/name (core 2 duo, Pentium, etc).
Also, you did not state what kind of motherboard slot you're using. Some only have AGP instead of PCI-X, or only have PCIx-4x instead of 8x or 16x.
Next Important piece of information missing is your Power Supply 12V Amperage. 12v info along with CPU series/speed will tell us how much power your CPU/MOBO use and how much will be left for your graphics card.
If you do not know this information or how to get it, try downloading and using a software Diagnostic. There are many programs that will tell you the brand of Hardware inside of your PC. The only one I am really familiar with is SANDRA ( sisoftware.net ). There is no guarantee that software can always figure out all the necessary information but it should give you enough to look up your hardware and find it online.
Please find your PSU model, CPU series&speed, and Motherboard # if you'd like a more serious answer. People are only guessing at this point because you're not really giving them any information to work with.