Welcome to AppleNova.
I hope you'll find our community helpful with easing the transition to your new Mac.
You sound like a perfect candidate for a Mac mini.
Both the iMac G4 and the Mac mini are very quiet. I've read that the iMac is completely silent during normal operation (less any soft hard drive accessing) but that the fans are relatively quiet when they do rev up.
The Mac mini is not snail, but the iMac will offer a substantial increase in speed in some cases. The faster bus and processor in the iMac can give it a great edge over the mini with intense multimedia work, but you probably won't be doing much of that.