The R9 280x tdp is 250W at full load. So with two that's 500W. I don't know what processor you are running, but an i7 won't break 90W. So a 850W PSU leaves you with 260W for the rest of the system. That's still a lot of room. That said, personally I'd get a 900W-1000W. It is overkill but that's what I'd do.