The 20" iMac G4 was new in late 2003 and was Airport Extreme compatible (802.11b/g). This means it accepts the current Airport Extreme card but I don't think it contained it as standard, unless it was the more expensive configuration. You can check if you have Airport Extreme already by opening the System Profiler application: go to Apple menu -> About this Mac and then click on More Info. It will tell you there.
This machine is NOT compatible with the original Airport card, which is no longer available from Apple anyway and is only 802.11b.
iBook G4s are compatible Airport Extreme cards (same as this iMac) but iBook G3s are not (old Airport).