The PowerBook G4 15" can be upgraded to 2GB by adding a 1GB memory module to the open socket and replacing the 256MB or 512MB memory module in the remaining socket with a 1GB memory module.

Standard Memory
512 Removable
Maximum Memory
Memory Expansion (No. of sockets)
2 Expansion Sockets
Memory Expansion
Install 1 at a time
Memory Sizes
256MB | 512MB | 1 GB
Memory Speed (Type)

Note that the 1.67GHZ 15" Hi-Res, model A1138, uses PC2-4200 Ram, all other models use PC2700 Ram.