Only half the iMac G4s had USB 2.0. If you have a 15" screen with a 1.0GHz G4 or better, OR a 17" screen with a 1.25GHz G4 or better, OR a 20" screen of any kind, then you have USB 2.0.
If so, then take a look at Macam (link), a free plug-in that adds support for most USB webcams to Mac...