Well, a quick google search tells me by default you already have an X800 XT, PCIe.
For anywhere between $60 and $80 you could go X1600 or (even better) Ge7600GS (or there are a few GTs this cheap, too). They have some ATI 2600PROs in this price range too, but they're the stripped down cards, I think. The 2600 also supposedly generates tons of heat and isn't the best card.
If you can move into the $120 range, there's the 2900GT. Sure it's got heat problems but it also has a huge fan and heatsink (on some models)
If you can go up to about $180, you'll find my wish-list card, the Radeon 3850 (or, hard to believe, even better the 3870 for ~$220)
