New and got low FPS


I don’t know if this is a bug and/if i can fix it easy but i am playing at Meduim setting and only have 14 FPS.
And when i swift to Low setting i get 15.
How can i increase my FPS?

My specs:
Interl® Core™ i7-4770 CPU @ 3.40GHz
12,0 GB
Windows 8.1
GeForce GTX 660

If i need a new graphic card i have been looking for a few things:
ASUS GeForce STRIX GTX 960 2GB PhysX
Fractal Design Define R5
Corsair CX 500W 80+ Bronze

Appreciate quick answers.