Some additional suggestions, for those of you that run Windows on the desktop.
- Get more RAM. Thrashing noises as the drive furiously flagellates itself coping with a Windows swapfile contribute excess random noise. Run as much as you can out of RAM, and your machine will be quieter. RAM's cheap these days.
- If you don't care how the machine looks, try using large slow-moving fans instead of small fast ones. Ideally, just embed an 8" or 12" desk fan in the side of the case and have it run just above idling speed. This will move he same amount of air for less noise.
- I found that gluing a sheet of soundproofing foam (the kind with the parabolic cavities on it) inside the front panel of my machine does a nice job of cutting noise without interfering with cooling.
- Of course, you could get a Powermac G4 cube. It doesn't have a fan. It's pretty damn quiet.