Go to the Apple menu and choose "About this Macintosh" and let us know what info you've got there.
The G3 iMacs (candy-coloured, CRT screen, handle on the top) used Airport (not Airport Extreme) cards. The very first generation of these needs a riser card installed to attach the Airport card to.
The iMac G4 (Dome shaped base with flat-panel LCD display on a chrome arm) switched over from Airport to Airport Extreme about halfway through its product life. If you have one of these, you'll need to click on the "More Info" button in About This Mac to find out what type of wireless hardware is supported.
The iMac G5 (current model. Flat panel LCD screen balanced on a steel base) uses only Airport Extreme, though recent models ship with this as standard.
Since Apple hasn't made Airport (non extreme) cards in a couple of years you really have to shop around to get one. eBay might pay off, though I managed to get one from an Apple dealer who still had a few stored.
Let us know what you find out and we'll point you in the right direction.
