Your motherboard will take any PCI-e expansion card, which covers anything made since 2007 or so. If you get the GeForce 8000, 9000, 200, 400, or 500 series or the Radeon 4000, 5000, or 6000 series it'll fit.
1) How much do you want to spend?
2) What Power Supply Unit (PSU) do you have? (or How many amps on your +12V rail?)
3) What CPU do you have?