All G4 iMacs are "Airport Ready".
The real issue - with the exception of the last two G4 iMac models which use the Airport Extreme card, all older G4 iMacs require the original Airport card. See the table at:
http://docs.info.apple.com/article.html?artnum=86258
The original Airport card is no longer manufactured or sold by Apple or it dealers. Only a few retailers still sell this card, and the current cost from them averages around $150. Ebay is now the best and lowest cost source.
As an alternative to getting an Airport card, I recommend use of a wireless bridge device like the Linksys WET54G.