looking at forums with a search engine:
For highest settings / high setting a r9 280, is good and less expensive than a r9 280X.
1920x1080 off course.
If going to a 4K display, 3840 x 2160, it is really a high resolution.
Testing 280X and gtx 770 at 4K on farcry 3.
First article looking on the web i see:
Review: AMD Radeon R9 280X vs. Nvidia GeForce GTX 770 at 4K
HEXUS / Tarinder Sandhu
http://hexus.net/tech/reviews/graphics/61201-amd-radeon-r9-280x-vs-nvidia-geforce-gtx-770-4k/If you wait till summer maybe you will find (r9 370 380 and 390 series)
and price drop for older cards.