If it's AGP, then one of the best you can get is an nvidia 6800GS. No matter what you get, check the specs carefully. There are both 128 meg and 256 meg 6600s, 128 and 256 meg 6800s, and there was even a weird 6800GT that had 256 meg of slower than normal memory.
Your cpu is fast enough to benefit from a good fast video card, so a 6800GS for around $230 is probably a decent match for your system. Newegg.com has one on sale for under $230 so shop around.