I am a bit paranoid, but my fans get really loud and I turn on the AMD Overdrive and set the fans to 65% so its quiet, and it runs on parts of maps or lets say sections at around 70-74c and then larger parts 80c. What should be the temperature that would melt me Graphics Cards is 80c too much? and what exactly is AMD Overdrive?
When I plan to play demanding games like Crysis 2 for more than half an hour I'm usually sure to set my fan speeds to 100% in the summer, though not as much on cooler days. As long as you aren't getting artifacts or crashing you're o.k. as 80c isn't too bad for a 5870. It's not great, but it's not a dire emergency either.
90c. You start going over that it's trouble. Anyway, I'd say set your fan speed for 70% and leave it there. My gigabyte cards are a little quieter than cards with a stock cooler though so I don't know if it will bother you or not ^_^