Hello, I own a MacBook Pro, and I wanted an extra Gb of memory to put in there, so I went to Crucial.com. They have a section to tell you exactly the ram you need. Plus, they are very cheap. I think my 1Gb strip came to £101 after delivery charges and everything were added on.
Apparently Crucial make their own memory modules and the ram itself, plus all the other mac users I know here in Bristol upgraded their memory from Crucial, so I had good reccomendations to make it more easy to decide.
Take a look on there, I checked for my friends old G4 Powerbook 15", but there were so many different options of that version of Powerbook, she needed to look herself to pick the right memory type. I would look for you and say "yeah go for this type of memory" but best to check and make sure you select the exact type of powerbook you have.
Hope this helps in some way.
Mel