I have two imac G5s and they are both fairly quiet. The early models apparently had some fan design issues according to what I've see posted on the net. But as far as I can tell, these problems have been resolved. Of course, it's all a matter of perspective. If you are running soft synths and electronic instruments, small amounts of ambient room noise are not a big deal. I also doubt that it would affect something like an SM57 on an amp cabinet. On the other hand, if you are doing voiceover work with a LDC mic sitting close to the computer, you will pick up some noise. The question is at what level and what are the consequences.
Having said that, It's worth keeping in mind that the computer noise (at least on the two G5s I own) is affected by the programs you are running at what you are doing in each application. My office machine, for example, makes more noise if I'm using an eraser tool in Photoshop to edit a large PSD file. The G5 in my recording room is new, so I haven't sorted out which operations tend to produce more noise. But, if I recall correctly, the noise is louder during playback than recording.
Ideally, the computer should not be in the same room that you're using to record. Of course, not everyone can have a separate room or closet for their machine. You can also build or purchase an enclosure. With the Mac Mini, building and enclose seems like a simple process. I'm not sure about the imac G5, since the monitor and cptu are all in one case.
Polaris20: If we keep talking about these kind of issues someone is going to come to the conclusion that the ideal Mac for audio probably doesn't exist! The Mini's drive are too slow, the imac makes too much noise and lacks open card slots, and the dual G5 towers (which are probably noisier than the imac) are too damned expensive!
