No graphic card has warranty for overclocking. If the card is damaged because of overclocking you might lose your warranty.
Sorry, but not true by any means. I know that XFX covers overclocking. I RMA'ed a card that crapped out on me being overclocked and I mentioned that I overclocked it sometimes and at the time it messed up it was not overclocked. As far as I konw XFX warranty covers everything as long as your board has all the original parts on it(no voltage mods, or aftermarket coolers). Murph