I have an nVidia Geforce 6800 and have been using RivaTuner for ages - its great for overclocking, unlocking stuff, generally good.
I read yesterday about how to manually set fan speeds, and if you are not already doing this (assuming that you have an active fan on it) then you must read this page here:
Bogeymans RivaTuner fan settings.
Basically, although my fan (an aftermarket Artic Cooler, but any work, even the ones that come with the card) always speeds up for 3d work, then back down again for windows, this allows you to set particular speeds for a given temperature. As a result, when i play Battlefield 2 my fan stays on only 40% speed or 55% speed, with temps staying between 50 C to 54 C (as reported by RivaTuner - different to what Speedfan reports).
I love this utility, and I love arctic coolers! I now can't hear my GPU fan even in full 3d gaming!!!