Ok IBM makes the PPC CPU right? Well heres a little lesson what Intel does with their CPUs. I think SUN and IBM does the same thing.

Intel basicly takes a CPU that was made for pushing 100MHz. They then take it and make it push 200MHz and lock it as the default clock. This is how the CPU runs very hot and needs a heatsink. Probly why the G5 has a big ass heatsink aswell. Basicly to save money from building a CPU some scratch they just take what the already have and modify it so it runs faster. All what overclocking does theses days is raiseing the FSB and the Multiplyer.

Heres how I see it. They pump the CPU with tons of caffine and when you overclock it you're making it drink even more caffine. If you go any higher it will go nuts and probly have a heartattack meaning the CPU will fry.

I remember reading a article that Intel's frist line of Pentiums were infact a customized 486 at 90MHz with a built-in 487 at 66MHz and more cashe built in. Then down the road it was overclocked to 166MHz then they made a new one from scratch and they built MMX.
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