The motherboard will work just fine with Crossfiring 2x R9 290!
What power supply do you have? I think you need at least 650-750W.
The R9 290's are designed for larger monitor gaming such as 1080p, 1440p and higher, If money Isn't a problem, go for the 4K or 1600p, otherwise 1440p. Actually 1440p should be best for gaming since it's smaller you'll gain a lot more fps
I have i5 4670K and 1x R9 290 on 1080p and I'm more than happy with the performance!
PS, do you already have the motherboard or are you going to buy it now?
-Nicholas