The X800PRO seems to be a bit of a underperformer overall, probably due to it's 12 pipelines (as opposed to the 6800 GT's 16), so it's pretty much an impossibility to OC this thing upto X800XT speeds (like you can with the GT to Ultra).
As usual, the X800s tend to perform better at Direct3D games like Half Life 2, and 6800s at OpenGL, like Doom 3, although the new Cat 4.9 betas have allowed the X800XT to overtake the 6800 Ultra in Doom 3 @ 10*7, *I think* 4xAA, 8xAF.
The more relevant argument lies with the 6800's hardware feature of Shader Model 3 though, that will tend to give the X800s more problems later on in terms of competitive performance.
However, at the moment I'm more than happy with my X800XT-PE, and SM 3.0 isn't a bother for me because I intend to upgrade again before Unreal 3.
EDIT: Oh shit, didn't realise I already replied to that quote.