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