I've been screwing around with undervolting/underclocking my CPU.
Right now im running at 0.85v, 800Mhz and the computer is perfectly usable. I'm also passively cooling it. Idel temps are 1C above ambient, load gets into the lower 30s (these are tCASE temps, not on-die, obviously). I think this would be the way to go for silence, though in a closed case a very low speed fan or larger than stock heatsink might be needed.
EDIT: droped voltage to 0.8, the lowest I can go. Very stable at 800MHz. Oddly enough, I can still play all my games just fine. FPS is significantly lower, but everything is very playable.