Is it normal for the fan on the ATI 5870 to emit a low grade buzz/humm at normal idle? Or should it be pretty silent? I had a ATI 4870 and the fan was way quieter on normal idle.
Your options in order of complexity, if your card is out of AppleCare, is to (a) take out the GPU and spray-can blast the vents in the GPU heatsink to get rid of dust, (b) remove the entire heatsink and clean it and the fan thoroughly, replacing the thermal compound while you are at it, (c) replacing the heatsink and fan with an OEM identical part or (d) replacing it with something else entirely such as a compatible Arctic Cooler triple-fan cooler for 5870.