I'd go with the Radeon 390 then. $270/290 after/before rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131672
390 and 970 are pretty close in performance, but generally the 390 usually barely squeaks by even at 1080p.
The latest GPU reviews with Witcher 3 are generally from the 380X which just launched.
http://www.sweclockers.com/test/21303-amd-radeon-r9-380x/8#content
http://www.computerbase.de/2015-11/...hire-test/2/#diagramm-the-witcher-3-1920-1080
Both of these sites show that a 390 just about equals a 970 in Witcher 3 at 1080. They also have aggregate scores showing that the 390 wins out on average at 1080 (and the lead slightly increases at 1440, which could be similar to 1080's performance in the near future). The extra VRAM is also attractive for future performance.
Your PSU certainly will have no problems, but be aware that the 970 will use lower power, at least while in a game (idle is usually about the same). If that is a concern to you then the cheapest 970 I can find is $260/$290 after/before rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127833
Another consideration for even less cash but similar performance in that tier is a 290 for $230 flat:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161459
It is essentially a 390 with half the VRAM and lower memory clocks (typically several % lower performance).
I don't think there is any need to go above. The 390X is barely faster than the 390 but asks for a mighty premium. The 980's lead over the 970 is a little more, but they also ask too much money imo. Unless you step all the way up to a 980 Ti that is.