All kind of overclocking does harm the hardware and shortens it`s livespan.
The only question is what can you do to compensate this effect.
For starters is might be a good to check if the graphicschips get`s much hotter then normal and then maybe add a cooler to the chip, but I don`t know if this will be a big help for a PLCC chip.
If you Overclock a C/BVisionPPC via CGX you MUST add a cooler and a fan to the chip (the tooltype to activate the overclocking for this card is I INSTALLED A FAN ON MY PERMEDIA spelled backwards)
I don`t know how neccesary this is for the CV64/3D but it sure wouldn`t hurt
