First off, don't get the 4GB 960, adding 2GB of RAM to the 960 is like putting lipstick on a pig, literally. Nothing the 960 is capable of needs more than 2GB VRAM.
That said, a 960 is not really a huge leap over a 750ti and now is not a good time to buy GPUs anyway as the next gen is coming soon.
Also, unless you are playing games that make use of additional cores, the 6600Ks IPC is not much higher than the 4170, so unless a game make suse of the additional cores, you will not see much of a difference.
The real question here is, what games are you playing, what kind of performance are you getting and what makes you think you need an upgrade?
How come that's the choice? You'd need a new motherboard (and probably some DDR4 RAM) for the 6600K, and that will push the upgrade price to well over £300.