I disagree about 2gb vs 4gb. Sure on most haha you won't see a difference, but I think as time moves on, you will wish you had the 4gb. As games advance and you want to turn settings up more you may find yourself turning things down to compensate. I know when I was shopping for games on steam recently some of them said if you have a 2gb card and attempt to adjust performance settings that you may hurt performance. So imo go ahead and get the 4gb if you can afford it.
Also, you might want to consider the r9 380 from amd. Performance is a little better than the 960 I think, or at least on par, and it's a little cheaper. But it will take a bit more power and may be a little warmer, but if you've got a decent psu and case fans in that area, should not be an issue. Personally at this point though I would not bother with a 2gb card unless there were just a very good deal or I cod not get anything else, because in the next year or 2, I have a feeling people will wish they had 4gb.