drm, Paul's account is pretty much right on the money. Having moved up from a dual 2.0 G5 to a dual 2.5 four days ago, I'll add my own observations for kicks (this is speaking about a machine that sits on the floor -- put it on your desk, and you'll notice the sounds it makes a lot more):
1) it is significantly quieter than the dual 2.0!
I was skeptical about this myself, mostly due to the fact that the dual 2.0 is so freakishly quiet while idle. Like Paul says, the machine only revs its fans while under load. For the types of load typical photographic processing requires the computer barely makes any noise. Instead, it "sighs" when you get it to do something, and then goes back to being quiet again very quickly once done. Even though the CPU fans are identical on each machine, this happens faster on the dual 2.5, and at idle the fans move slower than they do on the 2.0 (the air exiting the case is noticeably warmer however, so you won't be sacrificing any room-heating capabilities by tossing the Quicksilver :.
2) I owned a QS dual 800, and my job currently sits me in front of a QS-2002 dual 1GHz all day. THEY'RE HORRENDOUS IN COMPARISON (sorry for shouting, but it's tough to be heard with one of those in the room)
I live in a loft, and there is absolutely no freaking way I would EVER have considered leaving my QS 800 turned on all the time, as I would have gone mad from the noise. The G5 is in a whole different league. Before I passed it along to its new owner, the dual 2.0 reported 23 days uptime -- I shut it down for hardware changes, and restarted it for updates, but it was effectively always on and made a fantastic file-server
3) If you want to hear a loud computer, give a G5 a reason to rev its fans to full speed.
These things can make a lot of noise if they need to, but that's the key -- they only do it when they need to (in drastic contrast to the Quicksilvers and their ilk). It appears as though the LC in the dual 2.5 is simply more efficient at dissipating heat than the monster heat-sync in the dual 2.0. I haven't heard the fans nearly as much with this machine, although my usage patterns haven't changed at all.. That said, the Apple Hardware Test runs the fans at full-steam for the duration, and the dual 2.5 makes slightly more noise under those circumstances than the 2.0 did (yes, I ran them side-by-side the day I got the 2.5 to check). Under normal use you'll never hear them go like that, however.
4) Want an apt comparison of noise levels to your QS G4? Turn it off and imagine you were still getting things done 2-3 times faster. The G5 is how computers should behave. You won't be sorry you upgraded.