If you bump the voltage you CAN hurt the card. The issue with over volting is you don't know when it is to much then the card crashes and is fried. using the voltage is a last step, last resort move for more performance.
do some googleing and see if you can find the max safe over voltage for your card. then follow the steps I mentioned and you should get a little more out of that card. Be VERY cautious when coming close to the high end of the over volt limit because the closer you get the more dangerous it is for the card to be there.