a brave man asks about mac on this forum... but from what I understand the Mac has no Bios that can be tweaked, its a different beast entirely. So your most likely SOL. I wouldn't recommend over-clocking mac systems anyway since they are mostly notebook level components... but supposedly It can be done, just Google it.. (as I just did).
The late 2006 iMac with Intel T2400 chip is another beastie altogether - if anyone has been under the hood and improved matters they're keeping quiet about it!