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

I mean come on - that's really tight, it's not the problem You might as well strip the RAM (less watts used) and video card, OS X doesn't need them either! If it's working fine at that speed, you might as well keep it there. I suggest there is something else in your house that's using up power unnecessarily.

JB