If you don't own a original Airport Card or you don't want to block your single USB port with a WLAN Stick, there is still an alternative: A Wireless Ethernet Bridge, which is connected to the standard ethernet port of your iBook. The device is driverless and configured via browser-interface. For example the Belkin Wireless G Gaming Apapter F5D7330 which was constructed for XBox or other Gaming devices and supports 802.11g standard up to 54Mbps.
There are two versions: The silver one with two antennas supports only WPA encryption standard, whereas the white one with a single antenna supports WPA2.
The Browser - configuration is a little complcated, but you can follow the instructions at AppleInsider. A Firmware Upgrade might be useful for optimal performance.