Thats hot for idle, what is it under load? The max temp for most GPUs is ~100C. Did you get this computer used? I assume so since the 8800GTS is a bit old. Try blowing the dust out of its heatsink, it should drop your temps a fair amount.
GPU-Z would probably be better a better utility to use to simply monitor heat and fan performance. Not because Afterburner can't be used in that capacity. It's just Afterburner has features novice users should probably avoid so they don't accidentally destroy their card.
81C is definitely hot for an idle temp. Heck, it's actually a bit on the hot side for full-load, depending upon location, ambient temp, and ventilation. Try running tests with the case closed, case wide-open, and case open with an oscillating fan blowing into it, then compare results.