As for GTX960 I would recommend a 4GB model. It won't benefit many games but a few already for sure like GTA 5 (probably). It goes up to 6GB for which you need a much better card but that suggest to me more than 2GB is likely at GTX960 settings. And of course for FUTURE PROOFING.
*tough call to recommend 4GB over 2GB as it adds roughly $40 (about 20%):
http://pcpartpicker.com/part/evga-video-card-04gp43966kr
So that's about $225 for a 4GB model and $185 for a similar 2GB model.
GTX970 examples:
EVGA:
http://pcpartpicker.com/part/evga-video-card-04gp43975kr
or
MSI:
http://pcpartpicker.com/part/msi-video-card-gtx970gaming4g
Those should be pretty similar in performance so maybe decide based on looks or investigate review further.
*Um... 4GB vs 2GB?
When Watch Dogs came out you could use something like a GTX680 (slightly faster than GTX960) and it would run fairly well for a minute or so depending on how much you were driving around then BAM. STUTTER!
What happened was the game used under 2GB (depending on settings) then more textures I assume were being loaded in as you drove around and that would rise above 3GB. When it required more VRAM than you had what happened I think is data had to be loaded from the HDD or SSD which causes a bit stutter fest.
Call Watchdogs a poorly optimized port which it was, but the point is that the reason is because the new consoles have more memory so there's a big jump in demand. For most games I don't think too much more than 3GB of VRAM usage will be typical as game developers need to keep in mind the hardware install base but it is definitely going up.
Vanilla Skyrim (no mods) at 1080p would rise up to about 1.5GB of VRAM, but with mods can jump well over 2GB. An old game yes, but many people still play it. It's conceivable that you can get 60FPS with a few mods but need more than 2GB but I can't be sure. It's only an example anyway.
More than 2GB for a GTX960 will definitely benefit, it's just impossible to say by how much, how soon, and for which games (aside from the few that benefit already).