I don't know a lot about voltmodding, which is one of the reasons I'm not going to do it lol. But anyway, even at my current OC, the temps never go higher than 70 in every game I've played so far, including Crysis. I'm pretty sure the GTX can withstand temps up to the 80s, so I know I'm well within the card's temp limit.
I can hit 3.6GHz there too, but it can't run 3DMark06 without crashing.
What else did you have set in your BIOS? Was your RAM auto and what was your RAM multiplier?