Another option (cheaper too!) would be to use one of the various usb wireless adaptors. I have an old g3 iBook and a D-Link DWL-G122 usb adaptor which works like a charm. The only real downside is that you have to deal with an adaptor sticking out of the usb port, but it'll probably save you a good few quid. When I was looking for an airport card for the iBook they were going for £80 odd.
I got a free wireless router and the usb adaptor from my isp (madasafish) when I renewed my contract and have been really happy with it.