If you have heard one of the new G4 Dual GHz PowerMacs at a nearby quiet Apple Store, they really aren't that loud. I mean, you can hear them and the fans inside, but it's not really a "windtunnel". 🙄
Also, one of the more recent MacWorld magazines (as controversial as some say on the forums) compared the new dual PowerMacs, all models, as well as some old model dual PowerMacs, and I think the new dual GHz performed tasks better than the old 1GHz dual with every, if not, almost every, test executed, from Photoshop, to other high level aplications.
Personally, I don't have either model, but from reading posts and reports here, and in magazines, and just from personal observations, I think the new PowerMac models should work better for whatever your needs are.