I'd say running at low nineties is pushing your luck, and under ninety is preferable and if you can go below 70 then you are a-okay. There isn't thermal guideliness available (that I know of) for Nvidia/ATi - cos inexperienced overclockers out here will used it to launch claims against companies going : "you said that this chip can run 100C, but why did mine burn out?"
I had a X800XT once that was running 90-95C on load for a long time, and that card is still working! So it's a tough one. But the basic guidelines above will steer you more or less in the right direction.
Ensure you have proper cooling in your case, with good air flow.