Your B&W G3 doesn't directly support an Airport card from Apple, hence no obvious place to put it. You'd have to get a PCI Adaptor, like this one from Farallon and one of their 802.11b (technical term for Airport) cards. It doesn't work with apple's airport card. You'll have to return that one, unless you're planning on getting a machine that supports it.

Sorry to be the bearer of bad news.