The term "quiet" tends to be subjective. Have a look around in other threads and you're likely to find both those who claim their MP is "completely quiet", and those who say their MP is noisy.
My MP (2.8GHz 8-core) has got 4 internal HDs, the 8800GT grahpics card, and 10 gigs of RAM. The Mac makes a constant sound, part of it is due to the 4 HDs, part because of the graphics card's fan, and part because of the MPs own internal fans. The noise is a humming kind of sound no matter if the Mac is idle or under heavy use. The temps never get too hot, and the fans run at their lowest possible rpms most of the time.
However there are actually times I can hear my MPs noise even when I'm using headphones, so... quiet it is not. When I put it to sleep, it is dead slient though.
There are ways of silencing the machine. I haven't tried any of those tricks yet, but I'll look into it. I've just been too busy working (ignoring the MPs noise) instead
Your other question concerning CS3: It works great. No problems at all, everything is rock solid and lightening fast.
The MP is a great machine, noisy or not. You won't go wrong with it.