Unfortunately, my X800XL doesn't do too well above 80C. Source based games seem to be ok up to 90C, but anything else (even Painkiller) either causes my PC to crash, or I get checker board artifacts. :x
on a hot day i can get 81ish out of my GTX(BFG OC)
Just know that your GPU can take ALLOT of heat and keep going.
My X850 topped 100(nothing like not seeing a line graph on ati tool)
My X1900 topped 90
They all ran fine like this. Do note my ATI's go after market heatsinks to drop the temps to the 60's.
I am not gonna worry about my GTX since its fan has never increased in speed(seems to always be @ 60% and thats quiet enough for me) yet(unless that bastard does it while i sleep....sneaky flucker).