A ATI Radeon 9800pro will beat the 6600GT, I think the 6600's bigger brother 6800 will beat the 9800pro.. But won't beat the X800 Standard one.
So to answer your question.. yes the X800 will be much faster.. 7600GS has much more features and is ruffly the same speed as X800.. So rather get that.. Dunno what prices X800 are selling for vs the 7600GS..
Just remember also, are you buying AGP 8x or PCI-express.. Cause Most X800's are AGP 8x, and the 7600GS is only PCI-Express as far as i know...
I know u get the Radeon x850 in PCI-Express
Hope this helps...