Since the CPU's voltage has not been increased, there will be no discernable difference in power usage at 400 vs. 300Mhz.

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Well, the power consumed by digital logic ("CMOS" to be specific) depends linearly on clock frequency, and quadratically on voltage. So, there is most definitely a difference in power consumed by the cpu at those two clock frequencies (which is why cooling the chip is sometimes a limitation in overclocking). But I agree that the difference is inconsequential as far as the overall mac's power consumption is concerned, because there's a lot of other stuff that consumes more.