Judging by the screenshots alone Iīd say the memory got damaged. If you donīt get such or any artifacts in other games, the chances are quite good that its a software/driver issue. For fixing this problem I would recommend to go back to default settings in Rivatuner and to a driver which you know that didnīt produce visual artifacts, or if everything fails, to re-install Windows as a last resort.
Good luck and welcome to NvNews!
Edit: When I think about it, Daydre@ms thermal/overheating suggestion sounds much more realistic then mine.