maxalge :
Martell1977 :
In terms of performance the list breaks down (fastest to slowest):
R9 390
GTX 970
R9 280X/380
GTX 960/280/285
R9 270X
I'd say that anything from the R9 380 and up should suffice for your needs.
fixed for accuracy
Except the R9 380 is a rebadged 285. It has an OC, but that only netted about 1 FPS.
http://www.tomshardware.com/reviews/amd-radeon-r9-390x-r9-380-r7-370,4178.html
The mistake I made was putting the 960 over the 380/285. The rest is accurate.
So ultimately:
In terms of performance the list breaks down (fastest to slowest):
R9 390
GTX 970
R9 280X
R9 380/R9 285/R9 280/GTX 960 (with the 960 a little behind)
R9 270X