Should be fine. PCIe 3.0 is twice as fast as PCIe 2.0, and PCIe 2.0 x8 wouldn't bottleneck a 7850, so you should be fine. I wouldn't recommend going dual 7850's, they aren't that great. I've done it before, and it was pretty crap. I spent more time configuring it, then actually playing games with it.
Well I am waiting for HD 9xxx or GTX 8xx, so if I can get a HD 7850 card for £100 it will be better than one HD 7850 that is doing me okay atm.