I've heard of a fan upgrade for those noisy G4's.  I have a noisy G4, but I have such severe tinnitis (ringing in the ears) that a little extra noise doesn't mean much.  I'm sorry I don't have specific info about the fan upgrade to cure the G4 noise.  I'd try some Mac forums for info.  I sure wouldn't go to the expense of a G5 just for the noise problem.  My brother is getting a G5 that should be arriving today.  I'll let you know about the noise thing.

He's getting the G5 not because of any noise issue, but because you need it if you want to use the 30" monitor.  The huge memory you can put in a G5 is also very nice if you want to have a large amount of stuff on the screen at once.  The extreme speed of G5 for processing intensive stuff like raw conversion is also nice.  I don't do much raw batch processing because I like doing individual tweaks like exposure, getting rid of vignetting and/or CA, etc.  There's a lot of possible situations where a G4 keeps you waiting a lot longer than G5, but that depends on what sort of work you do and what file sizes you work with.

Maybe the best place to check about the G4 quiet fan thing would be  Here
It's a site for pro digital audio and their forums have info about this.  Go to "Support" in the top menu bar and then to "User Conference".  This is where I first heard of the quiet fan upgrade.

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.

Here is a review on the G5 that appeared on the PC Magazine website.

Hi,

I'm on the verge of buying a Mac G5, because the noise that my G4 makes is driving me insane. This is a "quicksilver" dual processor G4, which doubles as a room heater...

Is the G5 significantly quieter ? If it isn't there isn't much point in upgrading - the G4 does more or less what I ask of it.  Is there any difference between the G5 specifications on noise ? I've been told that the 2.5Ghz is quieter than the 2Ghz due to the liquid cooling system, but I'm a bit skeptical on that.

Real world experience gratefully received!

David.

I upgraded from a mirror-doors dual-1 G4 to a dual-2.5 G5. The G4 had the Apple power supply retrofit installed for reduced noise, which really did help a lot.

The G5 really is much quieter when idle. Not silent, by any means, but quieter and also more mellow-sounding.

Both machines will speed up the fans when needed. My experience is that the G5 fans speed up right away as soon as any demand is placed on the processor. Launch Photoshop, and the fans spin up. Let the machine fall idle, and pretty soon the fans go quiet again. This can be distracting and takes some getting used to. With the G4, the fans didn't spin up until the machine had been busy for a while, and kept running for quite a while, so it wasn't so obviously triggered by my actions. Somehow this was less distracting. The fan transitions really bugged me for the first few days with the G5, but now it seems normal.

Both machines get pretty loud when the fans are on max. The G4 would always start up with max fans, and then throttle them back. The G5 is smarter than that, so I only hear max fans if I let the room get hot and keep the machine busy.

I have not had a good opportunity to compare the 2.5 G5 to a slower G5. Listening in the Apple store (a noisy commercial environment) was inconclusive. I don't believe the liquid-cooled model is likely to be significantly quieter. It still relies on the fans to get the heat out of the box.

 -Paul

David,
Here's my experience with dual G5s.
I'm a very noise sensitive guy. After having heard a QS G4 for about 30 minutes in my office I decided it would be sent into the server room and stay there forever. It received the so-called silent power-supply kit but it remained a very noisy machine.

I used an iMac G4 and it was a very quite machine, just not fast & expendable enough. When the iMac G5 came out, I immediately bought one and had only problems with it. In fact, t was a very noisy machine (I was a victim of the "whisper-quite" marketing stuff), power-supply died as well as the hard drive. Mother-board had to be swapped by me and by the repair center and it remained too noisy for me. I produced a high-pitch noise in synch with the CPU load. Just think of a radio controlled airplane model. It drove me crazy and in the end I dumped it. The guy who bought it is happy with it and seems unaffected by the buzzing noise.

I had the opportunity to work with a dual G5 (1.8 or 2.0 ???) in a very quite room and it was a revelation. Sure, it's no PowerBook but it sounded much like my iMac G4.

I immediately ordered one and after a very long wait (Thx SwissPost for loosing it) I finally enjoying a quiete Mac. I didn't order the Dual 2.5 because I was quoted a 6 week wait at least before delivery (I understand that at this time you can get one much faster). I was prepared to buy some sound insulation foam but it looks like it won't be necessary. I was also ready to drill the wall to put the G5 in another room as an extreme measure...  
I can't really comment on the relative noise level between the non-liquid cooled Macs and the dual 2.5 as I've not worked with both machines at the same time in the same room.

Francois

Thanks everybody - very useful input.  So, G5 or ear defenders then :-)