I have both a iMac and a Mac pro:
The iMac is super quiet - 1/10 on a noise scale. The Mac pro more like a 5/10. Quieter than my old windows PC's though.
For me it wasn't the sound of the machines, but the overall heat produced. The iMac isn't very hot at all... i'm very impressed. My MacPro was a toaster oven

Ended up frying my ati 1900 card. Waiting for a new one to be delievered.
If they put a kick ass graphics card in the iMac, I'd say it would it would be a pc slayer and on a lot of christmas lists. As it stands, it's average but a great machine and experience. It's funny how $100, if that, could make the difference in a graphics card, but apple chose to not put in the top card for some reason. My guess is heat. Form over function wins again in this case