I had a G4/400 PB and now have a 12" G4/867.

The 400 runs warm and it only gets hotter with faster processors.The 867 can get hot enough to literally cook an egg on, but surprisingly, the fan only comes on when the vents are obscured. Another useful thing is that the fan speed on the 867 is variable, so when it does come on, it's not too distracting. The 400 got annoying when it would heat up, turn on the (noisy) fan to cool down, then start the cycle over.

When charging, the 400 does get really hot around the top-right corner where the power supply (and the power button) is. The 867 gets even hotter.

Apple have just dropped the US prices on both models as it looks like the 12" is due for an update and the 15" is likely getting replaced with an aluminium model.