Overclocking Macs is not really a subject you read about very often. Mac lacks a BIOS setup and Windows overclocking tools will not run under Mac OS.

That’s why ZDNet.de developed the overclocking tool “ZDNet Clock” for Mac OS X. The current version 1.0 supports only the Mac Pro and the Apple server Xserve. An Intel processor and Mac OS X 10.5.x Leopard, are also required.

The step-by-step procedure on going about it can be found here.