If 100% makes the fans too loud for comfort, try the tests using your normal gameplay fan speed (mine is 60-70%) and see what temperature you get.
70C is a safe maximum temp, but 80C for short bursts wouldn't do the card any harm, just not over prolonged periods.
A 20 minute test would give more realistic readings. Keep an eye on the temps if you run it for this long, and shut it down if you see artifacts or if the temp starts to get too high.
You could also use ATITool or FurMark which both allow tests for overclocking.