I wouldn't Crossfire a pair of 290s on 850w, that's pushing it close. The 290 and 290x can draw a lot of power at peak loads. 1000w is the general recommendation for CPU/GPU overclocking on a pair of 290s.
Further to the 290 Crossfire recommendation, you'll want a much better board for that. The PC Mate only has 1 x16 PCIe 3.0 slot, the other is a 2.0 running at x4, which will hinder the 290 pretty severely.