with the cyberstorms I have read that the 604e/233 uses 2.4volts and the mach5 models (from 240 to 400) between 1.8 and 2.1... The 060/040 use a voltage regulator, do you know if the ppc has another one? If there is such a component I guess it would be easy to change it for the right one... and get a 400Mhz CyberstormPPC (if the multiplier allows that, of course...)
