Before I gave my system to my sister (and went to my backup system), I had a watercooled Duron [email protected]
The heat exchanger I used was a heater core about 6" X 7" X 2" with 1/2" barbs I epoxied on. I kept it outside of the case with no fan, and positioned it so that the fins were vertical (I figured hot air rises, so I had to keep the area above and below it clear), and the temps at load wouldn't go over 43C. I was shooting for silence, but I never got around to changing the fan in the power supply (Sparkle 300 watt, BTW), so I never did get it silent. I honestly think the "Noise Killer" sticker on those power supplies is bull$#!+.
Anyway, having silenced the fan noise in my backup and my POS backup, it is now the hard drives that are the loudest, most obnoxious component. I have them rubber mounted with an attached ground wire, and moved to the back of the case, but they are the last thing between me and silence. I'm considering laptop hard drives, but they are expensive for the size that you get
(sigh)... So take my word for it: it never ends.