Hi,
There is no "best" GPU for any CPU. Whether the CPU bottlenecks the GPU depends on the game you're playing.
For example:
Battlefield 4 in Multiplayer is very demanding of the CPU. But,
Tomb Raider is not very CPU intensive.
I don't disagree with the GTX960 since I don't know what games you play. It's also worth nothing that future DX12 games should reduce the CPU bottleneck of your system by quite a bit meaning a better GPU wouldn't be as prone to be bottlenecked by your current CPU (for for all games, only DX12 games running in Windows 10).
The R9-280X 3GB is also a great choice, however they're more expensive on average and the cheapest ones may have some issues. My choice is:
EVGA GTX960:
http://pcpartpicker.com/part/evga-video-card-02gp42966kr
**WARNING:
Free game promo ends soon (tomorrow):
http://www.bhphotovideo.com/c/product/1115847-REG/evga_02g_p4_2966_kr_geforce_gtx_960_ssc.html
So if you buy TODAY make sure you know how to REDEEM the code ASAP!
(I think it's sent automatically to your address after you buy it, then you redeem the code once you get it.)