The G5 was PowerPC was it not? It was the first Polycarb Intel iMacs that used the Core2Duo in 06/07. And wikipedia lists them as using T processors, which I think are mobile chips. You might be able to get away with dropping in another 65nm T chip with a similar FSB. But the highest clocked with 667MT/s FSB is 2.33GHz. Then you've got to consider if the cooling is adequate for a higher clocked chip, though wikipedia says the voltage doesn't increase... and that's provided the chip isn't soldered/glued to the socket or anything.
All in all it seems like an (expensive) hassle.