You may want to think about hanging on till April 1st, when Apple is to announce new products. The likelyhood is that the new iBook will be out then, with a Core Sole 1.5 Ghz (guesses based on new Mac Mini), and DDR2 memory, which will make the whole system much faster. A SATA hard drive at 5400 RPM will also improve speed (again, speculation, as that is what the mini now has).
Personally, I would wait, as I have an iBook G4, and its fine, but really, not very fast