Both cards have an edge, as we saw the 6800 killed the ATI card in the Doom 3 tests, but it might be the other way around when it comes to HL2. But I think the GF6800 might also have an edge in future SM3.0 games, since the 6800 has more support for it.
On a website I recently visited, Gabe Newell quoted that ATi's x800 was running HL2 at least 35% better then the 6800. So yeah, both cards have an edge, ATi for HL2 and Nvidia for D3.
Doom3 played flawwlessly with my 9800XT at full settings, no lag ;D. Liquid cooling with the video card lets you overclock really high. And there is no danger to hot days in the summer and running D3 or HL2 at your computer quitting.