Interesting, I'll test my MSI 8800GT OC today with Rivatuner for fan speeds. Right now from 25% (lowest) to 30% I can tell the difference with my case, and 35% is borderline annoying. Maybe I'm really sensitive to sound but other people have commented that my computer is fairly quiet.
I may end up putting off OCing though if it's similar to the 7800GT...ah well, factory OC is not bad either =)