Nobody is making G4's anymore (for years now), and I think the fastest of those upgrades are just overlocked G4 CPUs. You would be better off with a newer G4 that had atleast 133Mhz RAM to stick a faster G4 in. I installed a 1.8Ghz Single G4 into sombodies DA and MDD and they felt faster, but the cost was not too cheap. The difference between a dual and single CPU depends on what software you will be using.