JMke from Madshrimps notoriety sends along word that he's found someone who's already benched a 7800GS against an X850XT & posted their results in their forums.

"From these benchmarks, the GeForce 7800 GS at stock speeds performs close to a Radeon X850 XT, usually being slightly faster in some tests, and slightly slower in others. It is only when the 7800 GS is overclocked where the performance is a bit better than the X850 XT"