You would need a terribly high power psu
The r9 390 is good for 1440p no worryes
http://www.redgamingtech.com/amd-radeon-r9-390-gpu-review-sapphire-nitro/
But not so much faster than the gtx 970 except if your game can support mantle
I have a gtx 970 because of the nvidia shield but if i wouldnt have i would buy the r9 390 no question
But not for the vram only for mantle