That calculator is useless imo. Its leading you to fear a bottle neck, which in reality is mostly a non issue, especially since you have a relatively capable intel cpu ( If you had an AMD it may be more of an issue). It also does not take into account some important factors, like overclocking!
As long as your cpu is overclocked to a reasonable level (4.2+), any bottle neck that occurs should be minimal/not worth worrying about at all, thats even with some of the higher tier cards currently available! This is especially true for the graphics cards you are looking at.
A 1060 (6gb) would be fine with your cpu, as would even amds offering (rx 480 8gb) that have a little more cpu overhead.
If you can afford a 1060 (6gb) or 480 (8gb) I would recommend getting either. (They are comparable in performance, although generally at this point, the 1060 is SLIGHTLY faster over all). Both should be faster than a 970 and will not suffer any issues from the poor memory setup (3.5+5).
If you cannot stretch to these, then a 480 (4gb version) maybe worth a look, but cannot recommend the 1060 (3gb version) as its not just the memory that been cut down (you would probably be better off with a 970 in this case).
A 970 is only worth looking at if there is enough saving over the above options IMO, especially if you are looking at second hand.
