Temerature-wise, the G5 is quite cool. The 2.5ghz 970fx's rated maximum power load is 50 watts, while a mobile Athlon 64 on the 90nm process runs at at least 60 watts. Compare this to the 3.8GHz Intel Pentium 4 Extreme Edition running at over 120 watts, and the G5 suddenly looks cold. The next revision of the G5 (both gx and mp) are designed to be even cooler running, with the gx at 14 watts at 1.8ghz.
I'm just trying to get people to see that their computers don't have complicated cooling systems controlled by more computers inside of itself and custom water cooling because their processors are flaming death-bombs of silicon, but because Apple didn't want to make an inelegant case.