I use ATI Tool on my 8800 GTS 320MB card. It's the most stable program I've used, and it also has a feature to stress the graphics card and find out if it's stable.
I use ATI Tool on my 8800GTS 320mb, and that works perfectly, it also has a stress tester, and you can just plonk in the numbers by hand, hit enter, and it will apply it, if it don't like it, it will artifact or crash!