I wouldn't upgrade from a 9800pro to a 6600gt. Ok... I lied... I did... but only because my new A64 chipset wasn't getting along with it anymore and I needed a different card but one that supported component out.
The 6600gt is certainly a faster card, but it's not worth an upgrade in my opinion.
I think a 6800 would be a good idea.
If you can find an X800XL in your budget, that would be a great card. It seems like you're going to need an AGP card though, and AGP X800XLs tend to be more expensive. The 6800 series seems like the better deal in AGP.