depends on what cooling you are using too. With water, you should be in the high 20 range to low 30 range(dependent upon ambient temps). With air, you want to remain below 45. Your cpu will run at 50, but that is where I start getting scared. 60 is possible, but if you are that high, turn your comp off before you fry your chip.
TT120, spode got an amazingly high temp i think in the 90C range and it survived, his is the only story i know of though.
Basicly, below 40 and you are good!