quarksoup: Lock to 30 FPS and make custom settings with a mixture of low- medium- high and you should be OK. I had a quick look and there are people getting 35-40 with your card on custom medium ish settings. It'd be recommended to lock to 30 though otherwise you'll get stutter from fluctuations.

Here is a great guide for tweaking settings. Basically some of the graphics options have a far greater impact on FPS than others. So turn hairworks off and shadows, foliage draw distance, ambient occlusion to low then you should be able to choose other options like texture detail and water quality to high or even ultra.

http://www.gamersnexus.net/game-bench/1952-complete-witcher-3-graphics-optimization-guide-and-performance

I used ths method to run the game when I had a GTX 770 and got decent performance, 50-60 FPS with the 'critical' settings turned all the way down and everything else ultra.