What RAM do I need??

I want to purchase a new 1gb RAM module for my PB. How can I know what I need, the thing is that in applestore they already sent me twice the incorrect one (after checking themselves with the technical department and giving my serial number!)...

thanx

Powerbook G4 1,67 GHz PowerPC G4 Mac OS X (10.4.8) 512 MB DDR2 SDRAM