eh. I'm not so sure its so clear.
The 6600GT is practically the same speed as a Non Ultra in Mid resolutions. At higher resolutions the 6800NU pulls ahead by a few percent. I have owned both. If you can mod the 6800NU, You will get about about 15% performance boost. Which is nice. I havent followed the mod luck lately.
The 6600GT does have advantage. Fully functional pure video codec and will do FP blending better than a 6800NU. It also consumes less power in 2d mode. Anyway. I have owned a 6600GT, (And SLI setup) A 6800NU (Its dead though
) and a 6800GT. And the 6600GT is so close to the 6800NU I wouldnt buy the 6800 unless you specifically are interested in modded.