I suggest non-X Sapphire Tri-X R9 290 } x2 in CF :)
Why?
1) R9 290X is not worth extra $ (30% more for 8% more performance? it's max 0,5-2fps)
2) R9 290 is cheaper than GTX 780 and it loves 4k more than his "green buddy"
3) R9 290 series doesn't need ugly Crossfire Bridge
But:
1) nVidia...