The iMac G4 is a fantastic looking Mac, if you are considering buying one I’d be tempted to go for the 1.0 GHz or 1.25 GHz models with either a 17″ or 20″ screen. The later models can be unofficially fitted with a maximum of 2GB of RAM, which is easily installed by removing the underside of the dome base. With maximum RAM installed Mac OS X Leopard should run reasonably well, but for best performance it may be wiser to stick with Mac OS X Tiger.
The iMac G4 doesn’t have internal speakers so look out for ones that come with a pair of external Pro speakers.