I have 6 fans in my case but......
I have an idle on an HD5970 of around 48C. When I play a demanding game I get a max of around 82C - 85C.
I have 2 questions to ask...
Should I manually put the fan to say 100% when playing a game?
Or should I just leave the software to decide?
generally leave the software to decide.
Those temps are high, but not dangerously high. Perhaps look into putting the case in an area where it can breathe easier, and make sure that it is not recirculating warm air into the case. Also, consider redoing the thermal paste as it may be dry and causing your temp issues, or getting an aftermarket GPU cooler to help calm things down.