I have both the iMac 2.1Ghz and a Dual 2.5Ghz PM.

Both run SSD's and both had their cooling systems redone. iMac cleaned properly and good thermal paste. PM's water cooling system totally rebuild with good thermal paste.

Both keep quiet most of the time, but the iMac will spin up it's fans when you push it for a while at max frequency.

The PM doesn't spin up the fans above normal speed no matter what I throw at it.

Speed wise the iMac is really fine for most things (it's got a better GFX card than my PM), but for CPU tasks, the PM walks over the iMac ( especially with things like browsing )

The iMac seems to handle the CPU scaling better (+- 45c idle) where as the PM pegs it at max frequency always and sits at a constant 71c.

If I can get the top of the range AGP video card for the PM I would be very happy.