With autodetect - 600/1100. Haven't pushed it yet...I'm sure it could do a little more..
No corruptions/artifacts and running cool (load temps of 42-43C according to Nvidias temp monitor).
Gainward Ultra/1960 XP Golden Sample (AGP)
The 6800nu OC is better, get that if the price difference isn't a factor.
I have a 6600GT @600/1100 myself and I'm happy with it, probably wouldn't trade it for a 6800nu since there are no guarantees that it will unlock (6800GT or 6800 ultra is another story however :D)...but that's just me...
I did almost the exact same switch, went from a 9700 non pro (flashed to pro@deafult pro speed) to a 6600GT. I didn't notice much of a change in HL-2, but there was some (like not getting the low dips I had been experiencing before and being able to run with 8xAF on). Most noticeable is Doom3...