As for the G5, moving 64 bit addresses around takes longer and used more power than moving a 32 bit address.
The memory bus is parallel, not serial. All 64 address lines latch simultaneously, ditto for the data responses.
As to raw bus speed, one of the big advantages of the G5 is that Apple has boosted the bus to a full 1 GHz. For a 1 GHz processor, this means a multiplier of 1:1 -- the ideal configuration. And for a 2 GHz system, 2:1 is still great. Heck, even 3:1 will still rock. Memory busses don't really become a bad problem until 6:1 or so.
I think a really advanced g4 makes more sense for a PowerBook. Maybe IBM will come to rescue as Motorola is certainly not going anywhere fast.
Capability-wise, the G5 can blow the doors off a G4. It's just a matter of the software catching up to it.
The real issue right now is heat and power. Apple went with Motorola's new G4-class processor this round because it uses less power than the previous rev. But IBM is catching up. Their currently-sampling 90nm (I think that's the right size) chips are showing great promise. They even have a variable clock rate - great for lower-power modes.
- Dan.
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