I'm an ATi fan myself, but for most folks (those without p4C 3.0 + or Athlon 64s) both the 6800(and upcoming 6850) and the x800 are going to be processor bottlenecked and produce very similar results. Comparing the Anandtec and Toms reviews It seemed to me the X800 was the clear winner on the Athlon 64 rig (Anandtech) and it was much closer on the P4c (Tom's). Bottlenecked on a P4C 3.2?
In AH1 and AH2 I expect both cards to produce frame rates grater than the monitor refresh (85 to 100) rate at maximum quality all the time.
With the games I play, it wouldn't make any difference which card I used. But when you consider the power requirements and heat produced (and left in the case rather than exhausted) of the nVidia 6800 the ATi X800 XT would be the clear choice. I'm going to wait a while and see what else develops with PCI-x, DDR2, new prescott steppings, etc. Then new rig time?