are you under warranty? because an overclocking of a computer usually cancels out warranties
i would not try to overclock your machine unless you really had the need to for some really important reason
my suggestion is just get a faster machine and leave your g3 400 alone since it is still a very good machine by most user's standards and overclocking it does pose some risk
the intel celeron is a poplar desktop machine to overclock since it's cheaper and easy to do without the risk of losing a tremendous amount of money but translating the overclocking hobby of a cheap pc desktop to a mac laptop is foolhardy for most users
