First the Mac G5 is very quiet, the multi-speed fans are staged so that the only time I ever hear the G5 running is when I'm taxing the whole machine to the point that it needs to kick in some cooling to disipate the heat. Doesn't happen very often, maybe once or twice a month. Of course I do have the dual 2.0ghz processors, with 1 gb of ram, and the standard ATA drives. If I upgraded to add more memory, I might eliminate even the once or twice a month, don't know, maybe I'll try that later.
As to machines, and I'll only list the ones that I have in active use.
Mac G5 dual cpu 2.0 ghz (also run the emulation Windows XP software for some tax programs for my option trading)
G4 Mac with built in 15" lcd screen (round one that is white).
2.6 Ghz Dell 17" screen laptop
1.8 ghz rack mount running Windows XP
(3) 2.4 ghz rack mounts running Fedora Linux
2.6 ghz rack mount running Windows 2003 server
1.6 ghz running Suse Linux
Xbox game machine running hacked Linux
TIVO running hacked Linux
Fortunately I run out of time, or I might have more.
Should add that the two desktop Macs do the bulk of the day to day work, although the XP laptop is the one that I am forced to use for work.