I bought a 80mm Vantech Stealth Smart Fan for my HTPC. I haven't installed it yet. It is suppose to be variable speed with temp sensor so it speeds up when it is hot. But I'm not sure if this is a function of the computer's motherboard or if the fan has the speed control built-in.
They are suppose to be super quiet and have ball bearings for longer life.
Its $10 at NCIX and $15 at other places I've checked.
http://www.ncix.com/products/index.p...inorcatid=1164
http://www.vantecusa.com/review/popr...h_casefan.html
Oh, and the 92mm version is suppose to be even quieter but I haven't found a cheap source yet.