First check if gpu usage during work in viewport is 100% and gpu frequency is at maximum. You can use gpuz utility. Many time is CPU bottleneck...it depends also if your models modifier stack is collapsed or not, if you are using proxies in viewport or full mesh models. Also check if all objects have backface culling disabled in obj. properties.
If you have all nitrous engine features on with scene lights and transparency ON, then go and buy 1070/1080. But I think 1060 will be also good option. How big are your scenes (interiors/exteriors)? What FPS is good for you?
Here is good table with all nvidia GPUs
https://en.wikipedia.org/wiki/List_of_Nvidia_graphics_processing_unitsYou can compare graphics cards according single precision column.