They run the fans slow as they can because they want it quiet. 80-90c should not hurt the card. my 8800GTX touched 88 on warm days.
A small bump in fan speed gives a large drop in temps.
@ 35% fan speed a 4870 loads(folding) around 60 vs 82. Its a little louder then i would like, but still not too much of a pain. For normal use the fan control is left on auto.