These machines are fanless and with an SSD completely silent apart from the optical drive. Anything that increases internal heat build up sounds like a bad idea to me.
Having said that, speeds range from 300Mhz to 466Mhz. If the chips are the same and speeds only reflect the clock speed each can run at without falling over then I assume overclocking a 300Mhz up a notch or two might be possible without too many crashes. Others might help you on that as I have never overclocked a PPC machine before.
Ok thx. I guess that a maxed out 466Mhz is the best you can get then. :/