Well, considering the fact that the G5 is actually kind of a "light" version of IBMs Power5, there's still way to go for that architecture before going to G6.
The Power5 is a multicore CPU, and the G5, in simple ways, is just one of these cores.
I think that the future for home and business computer CPUs will be multicore, the MHz myth coming definitely to an end because of it's technical limits (getting too warm and a high power consumption).
Intel's HyperThreading is just a pseudo dual core, but a real dual core would work like 2 real CPUs, not imitating them.
Apple already is prepared for that, as their systems are avilable with 2 CPUs since a while already. Mac OS X, as it's UNIX based, can handle multiple CPUs far better than Windows, as it was never meant to be used for more than one CPU.