That kinda artifacting is in practically all cases caused by 1)overheating and 2)GPU damage, in most cases due to overheating. I would try stock speeds and then see if it still does that. Also, try using some program to monitor your GPU temps, I can't think of one right now (except for CCC, there should be a temp monitor in "Overdrive").
EDIT: I wouldn't worry, though, I htink it's extremely unlikely that yo've permanently damaged your card, dialing back the OC should do the trick.
ANOTHER EDIT: Having a more careful look at the picture, I still think it's artifacting but there's a possibility of software error... I remember textures appearing weird in a game, something had damaged the installation and reeinstalling the game helped.