Not only will you damage your card, but your pc too.
If you look inside your pc case, are there alot of dust on the fans, fins and components? If yes, then it could all contribute to overheating.
I usually and very carefully vaccuum mine every 2 months.
But I think you may have a faulty graphics card.
Something similar happened to me when a stores name which contains 4 letters and begins with "A" sold me a faulty 8800 series card which looks like it has been used and resold as new. It was so hot that all the plastic near the card slots felt like they where going to melt.
They exchanged without a question.
I think you need a new graphics card.
If you have access to a different graphics card, why not test it in your system before jumping out to buy one?