I've got my 8800gts 512 running at: core/shader: 777/1944 and the memorry at: 1080mhz, With my e8400 @ 4,2ghz i get around 14700 in 3dmark 06, maybe you can clock the card higher than this, i didn't actually tried, but 1080 really is the limit for the memorry, I tried to get it up to 1100, wich was stable during some games, but in 3dmark06 with the deepfreeze test, it showed artifacts, so no go there.....
Anyway, mine doesn't get higher than 58 degrees celsius during 1 hour gameplay of crysis! (the fan is set at 80% or so) So temp is no problem with this card!
I've seen reviews where they were able to get the gpu to 820mhz! but i didn't tried it with mine, but it is possible, aspecially with a volt mod, although i wouldn't recommend volt modding, i never did it, and i'm not going to, just to risky, i love my vid card to much to screw it up
Anyway, 777/1944 and 1080 for the memorry is very stable!