The G4 iBook has one memory socket and can be upgraded using a 256MB,512MB or 1GB DDR module

Standard Memory
512MB soldered on board
Maximum Memory
1.5 GB
Memory Expansion (No. of sockets)
1 Expansion Socket
Memory Expansion
Install 1 at a time
Memory Sizes
256MB | 512MB | 1GB
Memory Speed (Type)

All models except the 1.42GHZ model use PC2100 ram. The 1.42ghz model uses PC2700 RAM.