Personally I think the G4 still has some milage in it. The current G4's (MPC 7455, ie Apollo) are still using a 0.18 micron fab process. When Motorola move to a 0.13 fab process there will be a further speed bump.
PS: The MPC8540 (Currently the only G5, or e500 core processer being produced) uses the 0.13 micron process.
Edwin