x2 280x Dual X or r9 290tri-x for gaming

Solution
R9 290, especially since two R9 280X is quite the overkill on a 1080p monitor. And single-cards are much more hassle free since:
you can't expect 100% performance in all games when you play in CF, it's much louder, much warmer and more power demanding going CF.

R9 290, have this gpu myself running 1080p, you'll hit Ultra in 99% of all games. You CAN go R9 280X CF for a higher FPS, if you want all the problems it brings. ;)

NiCoM

Honorable
R9 290, especially since two R9 280X is quite the overkill on a 1080p monitor. And single-cards are much more hassle free since:
you can't expect 100% performance in all games when you play in CF, it's much louder, much warmer and more power demanding going CF.

R9 290, have this gpu myself running 1080p, you'll hit Ultra in 99% of all games. You CAN go R9 280X CF for a higher FPS, if you want all the problems it brings. ;)
 
Solution
1 R9 290 Tri-X because it will handle 4k quite well and should be better than two R9 280X because CF optimization is not so good as SLI.
And if you have money for two R9 280X better to get single R9 290X because non reference R9 290X costs less than two R9 280X