To elaborate, its not actually the physical size of the chip that matters as it is the number of pins (copper connectors) on the chips, and how close together they are. The problem is that they wouldn't match up with the pin configuration on the G4. The problem re: the G4 iBook is the speed of the RAM. The G3 iBook has Single Data Rate (SDR) memory while the G4 iBook has Double Data Rate (DDR) memory.
- Some good news is that SDR RAM for your G4 desktop is cheap enough to buy without breaking the bank, since you can't use the iBook G3 memory.