haven't tried to really push mine yet but i ran it at core 600: mem 2000 for a while when i first got it to run some benches, it was stable though. Plenty fast at default speeds so dropped it down for 24/7 running. Maybe i'll give it another go when we can actually clock the streamprocessors or when some really taxing games come along ..
Used Riva Tuner to overclock my 8800GTS to 600/999 from 513/792, confirmed it with nTune. (would've been funny if I could get 666/999 ) Damn these cards make gaming look good, noticeable diff from 7900 series.
TJS
(I know you asked about GTX's, just thought I'd put in my GTS numbers...)
One of the main reasons I decided to upgrade was IQ, which was a higher priority in my decision than actual FPS increase (which is also uber nice ) hehe.
Can't wait to see what games look like on this card, gonna give the CellShock demo thing a try when I get everything installed, that looks cool as all get out