Similar thing has happened to me. I personally think the memory in the graphics card got corrupted. Simple fix would be to get a new card, I am looking at the 9600 for price/performance.
well glad to know 9600 is good. I may just go ahead and upgrade it this week. I need a new processor though, my 4800 dual core isn't helping much on some games, heck a 3 year old game sputters sometimes (Just Cause) and it's annoying me