Its all about the 6800nU/nGT - about the same price as 6600GT but much better performer.
Assuming we're thinking of the same cards, it's not a better performer and it's also pricer. I just looked at a plain jane 6800 and it was $270 compared to $215 for an eVGA 6600GT...
On top of that it's only utilizing GDDR2 memory instead of the new memory, both have 128mb of it and after all that I believe most benchmarks show the 6600GT to outperform the 6800
IMO you'd be better off with a 6600GT than a 128mb 6800