The easiest way is to increase the FSB speed (in very small increments), as the multiplyer for the chip is probably locked. I'm more of an Athlon man myself, so hopefully someone else (NeoMoses?) should be along soon!
You'll find the limit pretty soon, when your PC no longer boots! Of course be aware that upping the FSB also overclocks the rest of the system, ie ram, pci and agp. There may be settings in the BIOS to underclock these areas in relation to your new higher FSB setting to promote system stability.