>> PowerMacs G5 run at full speed. PowerBooks, iBooks and Minis do not.

> May I ask why? I believe on MacOS X the G5 powermacs are NAP,
> but the G4 ones not.

Because on MorphOS, Nap support probably isn't implemented for G5 (yet?).
Regarding Nap support for PowerMac G4 on OSX, the situation is more complicated apparently:

http://www.xlr8yourmac.com/feedback/MDD_CHUD_feedback.html