There are some pretty good benchmarks here:
X800XT v 6800 Ultra
Only read about the 3DMark scores and it basically says that in shader 1.1-1.4 68 Ultra is faster, but in shader 2.0 games X800 is faster, unless you OC the Ultra to 450MHz.
Another article I read suggested that ATi were almost hitting their limit in terms of headroom with this card, whereas the Ultra being much more efficient, has much more 'headroom'.
Its an old article but seems to make some sense:
Headroom?