The actual RAM is advertised as "for the iBook G4." I think that the reason that they say 640 is that there were no 1 gig chips that were slim enough for the iBook's slot. Since DDR SODIMM comes in different thicknesses, and the iBook can't handle the highest thickness, there was no 1gb chips that were thin enough. Trans Intl. has released a slim enough chip to fit in the iBook G4, it was advertised on SpyMac and some other places as well. It's ridiculously over-priced (Like $450) but if you need that RAM...(I don't)
BTW: MacTracker is a really great app, I've been using it for something like 3 years now.