old iBook = Airport = 802.11b = 11 Megabit
new iBook = Airport Extreme = 802.11g = 54 Megabit
A new airport extreme card wont work in the old iBook, and an old one wont work with the new one.
I suggest you look on ebay or goto any computer store, you can either get an old airport card from an apple shop of some kind, or get a usb device (probably cheaper), which will do the same job but will stick out of the side (ask if it will work with your iBook, or just bring it in and test it in the shop).