instructions on how to reset the PMU on the iBook G4 are [http://support.apple.com/kb/HT1431|here]
to [http://support.apple.com/kb/HT1379|reset the PRAM], reboot your machine while simultaneously holding down the Command, Option, P and R keys
one other suggestion (no idea if this would affect your problem, but just throwing it out there...) - double check you've installed the correct type/speed of RAM. for your iBook G4, it should be PC-2100 DDR226 SO-DIMM. apparently this model will also work with PC-2700 but that will still only run at 266MHz.