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.