The GTX960 can handle Vanilla Skyrim fairly well though your CPU will probably cap you near 60FPS.
However, when you MOD heavy with graphics you'll likely end up with a GPU bottleneck and even the GTX960 (or GTX980 even) won't handle some mods without dropping you below 60FPS.
I have an i7-3770K plus GTX680 (my 680 is similar to a 960). I tried some graphics mods but they varied a lot. Some just killed my frame rate though it may not be noticeable in all areas. For example, some vegetation mods dropped me from a solid 60FPS down to 10FPS in some areas dense with foliage. Yuck!
Bottleneck?
Again, probably CPU for Vanilla then probably GPU when modded but hard to say exactly.
I suggest looking into forcing Adaptive VSYNC. Aim for...