The one at work is my personal heater. The one here at home lets out a good amount from under my desk. I remember one day i closed the door by mistake, scared the hell out of me, there was a ton of heat in there.
My 2.66GHz Xeon MP with 6 of the 8 available RAM slots filled with Apple RAM and 2 Seagate 250GB hard disks runs fairly cool and does not output as much heat as my 3.0GHz AMD Sun Ultra 40 M2 that has 4 of the 8 available RAM slots filled with a 250GB WD HD under similar loading scenarios (running Ansys and ProE simultaneously). I'd say that the Sun's air output is 15-20F hotter than the MP, granted that my Sun has a Quadro FX 5500 card in it that probably runs hotter than the 7300gt card that i have in my MP and that the Sun's air flow isn't as efficiently designed as the MP's.
Is there a reason why you're concerned about heat output? I'd be more concerned with how hot the internal components of the computer get as opposed to the heat output.
If you were to get the MP, you can always configure the fan speeds with SMC Control. You won't go wrong with the MP. Good luck with the decision.
I have a MacPro 2.66 with 6 gigs of ram and 4 drives, along with a 23 ACD. If I close my office door when I leave, I notice that my office is at least 10 degrees warmer than the other parts of my home. Nice during winter, but it really noticable during the summer. I think all professional computer desktops are going to put out heat.
Almost 2 months back I switched up from an old G4 MDD PowerMac to a (see signature) MacPro. The old G4 put out HEAT - it was steaming after a day's work. This thing is room temperature on all sides except the processor seide which might be 75-80ºF... I can't properly tell, but it is warm to the touch on the processor side, and a far cry from being hot.