Intel Celeron 700@966 (92Ã—10.5)
Intel Pentium III 1 GHz (Coppermine) @1.2 (after a complex voltage mod performed by one of my friends, 160Ã—7.5)
Intel Pentium 4 1.6A@2.4 (150Ã—16)
One of the least impressive: AMD Athlon XP 2000+ 1.67 GHz @ 1.75 (140Ã—12.5)
It's as simple as the BIOS setup - in which FSB modes are selectable, under the CPU submenu usually, however, if you have a motherboard which cannot overclock with special BIOS modifications (which are unsupported), such as the ECS K7S5A, you may use tools such as CPUCool to make software FSB adjustments.
Usually, in situations such as this, you would be out of luck, and generally most people with say, Compaqs realize that overclocking isn't that big of a deal, but as I said above, try the FSB throttling/adjusting feature of CPUCool.