If you're talking an iBook, it's like most notebooks and kind of hard to upgrade hardware wise - not to mention the cost. You might be able to find a recent version, Apple factory refurb from Apple with same warranty as new for about $800. They usually can be found at Apple.com. clcik on Store and scroll down ring to Special deals. Then scroll through the refurbs until you get iBook.
Using OSX 10.3.9 on a G4