A 6850 -was- fine 4 years ago (DirectX 11, Ultra):
But as time went on and Blizzard kept milking their cash cow while piling more crap onto an antique game, it became not good:
And now 2 years later than that it's even worse, so no, a HD 6850 is not sufficient. If you want a recommendation for a card, read the STICKY
and fill it in, along with your budget.
AMD FX-8350 w/ Corsair H105, ASUS Sabertooth 990FX Gen3/R2, 8GiB G.SKILL DDR3-2133, XFX HD 7970, 512GB Vertex 4, 256GB Vector, 240GB Agility 3, Creative X-Fi Titanium w/ Creative Gigaworks S750, SeaSonic X750, Corsair C70, HP ZR2440w, Win 7 Ultimate x64