The G4 (
7447A) running at 1.33GHz has a 'typical' power consumption of 18 watts, at 1.42 GHz it has a 'typical' power consumption of 21 watts.
The G5 (
970FX) running at 1.4 GHz has 'typical' power consumption of 12.3 watts, at 2.0 GHz it has a 'typical' consumption of 24.5 watts. (( Above 2.0GHz is where the PPC970FX starts to suck power like crazy and produce tons of heat. ))
The issue isn't heat or power consumption. It's supply. IBM is having problems supplying G5 (PPC970FX) chips. It's a well know issue right now. When they figure out the supply issue, we'll see the G5 based laptops from Apple.