This was discussed to death when the G5 was announced. The bus is a major sticking point (not just speed, it talks a whole different language between the CPU and the northbridge). Also the memory bandwidth of a G4 (which is limited by the G4 north bridge) would really slow down a G5 chip.
Basically it's probably not worth it. I'd hold out for a dual-core G4 upgrade somtime in 2006 (maybe even a dual dual-core upgrade?)