I love my 1.6 GHz G5. It's VERY quiet. I can only hear a faint hum during use, and with music playing at even the faintest level, i can no longer hear the hum.
Things seem very zippy, but this is my first mac, so it may be that im just not used to the speed of os x.
I would say, unless you see yourself needing to upgrade to 8 GB of RAM ir using PCI-X, get the 1.6 GHz model. If you have some extra cash, maybe get the 1.8 GHz model, but i think that you will be quite happy with the 1.6