If you want the toughest test for your GPU, get this one:
It will load your gpu like there's no tomorrow. The temps rise higher than w/ furmark and in addition it reports errors/artifacts.
My GTX 480 can pass furmark at 850/1700 1.075v, but it OCCT it reports errors after only 2 minutes. You need to make sure your OC is perfectly stable. If your card runs a 30-40 minutes OCCT max stress test with no error, then you might consider that oc bullet proof.
My max & perfectly stable oc with OCCT is 800/1600 @ 1.050v and that proved rock solid in every game.