Let's refer to the Rojackpot.
GeForce 6800 Ultra Extreme
450 MHz
7200 MTexels/s
256-bit
GDDR3
550 MHz
35.20 GB/s
Radeon X800 XT PE
520 MHz
8320 MTexels/s
256-bit
GDDR3
560 MHz
35.8 GB/s
In theory, the X800 XT PE has the edge. You'd wana check benchmarks to be sure, though. Also, the X800's don't have Pixel Shader 3 support, so any game that comes out which uses it will end up being faster on the 6800, fillrates be damned.