from personal experience......
if you are running hardware at its max speed or even slightly past its actual max speed, you can "burn down" its ability to OC.
this can be true for CPUs, memory, motherboards, vid cards, etc.
its one of the main reasons why we preach "stability, stability, stability" here cause you can hurt the hardware if youre not.
dont run max speed ~ back it off a little (1 or 2 fsb on the same voltage)
also ~ bios updates sometimes lower the ability to overclock. you need to retest after updating the bios.
*Abit IC7 is a good example. bios14 can go 3fsb further then bios24 on the same voltage.