That uses an AGP slot. None of the cards you find will be good by today's standards. As far as old games go. I'd look at an ATI Radeon X800 XT, Radeon X850 Pro or XT, nVidia GeForce 6800 GT or Ultra or GeForce 7800 GS. Make sure they are the AGP models and at least 256MB. Most should be available pretty cheap on eBay.
If you are willing to spend the money. You can buy the AGP edition of the Radeon HD 4650/4670 1GB. They are probably the fastest AGP cards you can find.
Edit: Avoid models for made/flashed for Macs.