Is it possible to install Mac OSX on an iMac (G4) running Panther?

The Future of G4 iMacs in the Age of Leopard

Unsupported Leopard Installation
The good news is that Leopard seems to be able to run on any Mac with AGP graphics built around a G4 processor – and even on the 2000 Pismo PowerBook (the first PowerBook with AGP graphics) as long as it has a G4 upgrade.

The bad news is that the Leopard installer refuses to let you install it on any Mac slower than 867 MHz. That includes the dual 800 MHz Power Mac G4 as well as Digital Audio and other G4 Power Macs that have been upgraded with processors past the 867 MHz mark.

Just put the Leopard disk in and follow the directions. No need to install Tiger first. You need to check that your G4 can support Leopard before you start.

@jaytkay beat me to it! Great job!

Panther is OSX 10.3.

You can just pop in the Leopard disk, as @missingbite and @jaytkay has said.