If you are going to get pci-e (doesn't sound like it), get an X800XL. If you are sticking with AGP, get a 6800GT. They can be had for similar prices, but the 6800GT should be slightly faster. The thing about pci-e is, an X800XL can be had for $50 cheaper than a 6800GT in pci-e. So it becomes the better deal.
Anyway, look around, get the lowest priced 6800GT you can find (should be $250-280 iirc).
note: There is a false "6800GT" on Newegg for $164. It has MUCH worse memory (128mb of DDR instead of 256mb of GDDR3) and won't be much of an improvement over your 9800.