In my experience with different cards,programs designed to test gpu's for stability are not really working well. They only heat the card much more then games will,and you risk bricking your card.
And at the end,you still won't know if your oc is stable or not
Best way to test are one game I came accros over the years which seems to expose gpu unstability really well and 3dmark06. These will test your oc much better then furmark or kombustor or similiar programs.
Game in question is heroes of might and magic V ( and sequel VI also )
It will usually crash to desktop within 5-10 minutes if your oc is unstable. This happened numerous times on different cards with different overclocks on different computers with different os, cards which were rock stable playing crysis,crysis 2,battlefield 3,call of duty,skyrim,mass effect etc..for hours.
As for 3dmark,just loop "canyon flight" test for 10-15 minutes. It will also ctd if your gpu is unstable. If you pass half an hour of looping that test and half an hour of playing heroes,I guarantee you that your card is 100% stable
But,be prepared to be disapointed
I can't really remember how many times I wandered if it's worth raising voltage,sometimes a lot,just so one game and 3dmark doesnt crash when everything alse works fine !Edited by digiadventures - 5/12/12 at 8:10am