I can give you almost exact difference...
Anywhere from 75% to 100% speed increase depending on the title.
The X800XL and the 6800GS are almost identical in speed. Testing the X800XL I found it to be almost 100% faster than my 9800 Pro across the board.
With the 6800GS though, a few games like DOOM3, PACIFIC FIGHTERS and RIDDICK nearly 200% faster due to ATI driver issues with those specific games.
So in the end like everyone said. You will be able to go up a res setting and add some AA and AF.