I've lately been skipping to the overclocking section on every review with an index, as well as reading forums posts, and it seems to me that in general, GTs overclock to 410-420MHz, while X800 Pros to 520-540. Let's examine that in terms of fill rate:
350 * 16 = 5600
475 * 12 = 5700
At stock speeds, the X800 Pro has a slight advantage.
410 * 16 = 6560
520 * 12 = 6240
420 * 16 = 6720
540 * 12 = 6480
460 * 16 = 7360
570 * 12 = 6840
(last two added for those who decide based on the "what if I'm unreasonably lucky and achieve the highest overclocks I've heard of anywhere" factor)
As you can see, as you raise clock speeds, the situation reverses, and the 6800 GT starts pulling away from the X800 Pro. This is because it has 33% more pipes, and thus gains 33% more fillrate per extra MHz. If we take a look at other factors pertaining to performance:
6800GT:
SM 3.0
Much better OpenGL performance
Somewhat higher performance per clock
Drivers and games not yet (fully) optimized for it -> plenty of room for additional improvement
Will with 99% probability be faster at Doom 3 and other games using the engine
X800 Pro:
3Dc
Will with 99% probability be faster at HL2 and other games using the engine
Thus, I believe we can safely conclude the 6800GT to be the victor, other non-performance-related things (such as power draw, noise, heat, Linux support, brand preference, etc.), aside.
Anything I missed?