first give us some more info, like all the specs of the computer
an is it a brand name or custom built
to overclock u can go 2 ways, FSB increase or Multiplier increase. FSB X MULTI gives u the speed of the CPU
fsb can be increased on any mobo usually, but not brand name as in HP, COMPAQ, IBM, DELL - there overclock locked.
start ur computer and right away start pressing DEL a couple times, u should get into BIOS, then in there u can find options for increasing fsb.
multi can be changed only if u got a athlon barton thats unlocked, or u unlocked the multi on older athlons, or u cant if its an intel.
probably not an option.
increase fsb and see how high u can run it with your computer running normally.
voltage comes into play later, anyway get off ur lazy *** and do a search on the net. - there are like a million sites on overclocking.