No one can really answer that for you.
Each card is different to the next, so what one person gets another may not. Just do some research looking at what sort of voltage is required and then break the OC down into small increments and work your way from there.
So basically, find a rough voltage you would be looking at to hit 850core, then start from stock (don't increase voltage yet), add 15-20MHz, stress test. If it passes, keep it on stock volts, add another 15-20MHz and stress test again. If it fails, increase the volts up to the next notch (voltage increases in certain increments, can't remember exactly what it is though).
Just rinse and repeat being sure to keep an eye on the temps of the card and once you hit 850 or are no longer comfortable with the temps you are getting, just set it to what you are happy with and enjoy!