As a FYI, all Powerbook G4 owners should check your system to see what your system reports as the RAM quantity. Many of the motherboards have a serious defect. Here is a link describing the PowerBook Lower Memory Slot failure. http://lowermemoryslot.editkid.com/ BTW, I got hit with this problem and my serial number was not recognized by Apple as in the affected set.
Please forward this PSA to Mac audiences.
Cheers, AZ