@ OP: You'll find that there are a number of people here who don't approve of/use Furmark for stress testing. Ideally the best way to stress your OC is by actually playing whatever game(s) you play. Just like you would with Furmark; increase your clock speeds a little, do some stressing (in this case, gaming [which i.m.o. is more fun than watching a fuzzy donut do backflips]) and if you don't have any artifacts/crashes/abnormalities then go ahead and bump up your clocks some more.
Edit: As others have said, OCCT is a great method of testing stability as well.
We kept our friends at bay all summer long
Treated the days as though they'd kill us if they could
Ringing out the hours like blood-drenched bedsheets
To keep wintertime at bay but December showed up anyway