I agree with Duke normally a GPU can go up to 140C then it'll start acting funny and chances are it'll burn up. But yerr i wouldn't allow my GPU anywhere near 60-70 nevermind 100C.
If you nosey on the manufacturers website chances are it'll tell you what temp it'll burn up on. But from my experience you still have some head room.
My 8800GTS freezes the PC when it reaches 90ºC...