Hey Cassie, it's not terribly obvious, actually. It can get confusing because the term "Airport" can sometimes be used interchangably between the original and the Extreme. Here's two ebay sales, purely for the purpose of demonstrating the differences between them:
The first, the "original" Airport card, which is compatible with the G3 ibooks:
http://cgi.ebay.com/Airport-Card-make-your-iBook-wireless_W0QQitemZ150093012345QQihZ005QQcategoryZ80060QQrdZ1QQcmdZViewItem
The second, the Airport Extreme card, that is NOT compatible with the G3s:
http://cgi.ebay.com/Airport-Extreme-Card-brand-new_W0QQitemZ270092351559QQihZ017QQcategoryZ86701QQrdZ1QQcmdZViewItem
[Note: I don't know anything about either of those sellers or auctions- they just came up when I searched in ebay.]
Basically, when you go on the hunt for the card, most places will refer to the "original" Airport card.