more $$$ = more performance. more memory doesn't equate to better performance on a GPU. a 4gb 760 is only slightly faster than a 2gb 760 at most, but it still won't beat a 770.
do not based a GPU based on how much vram it has, as you still have to consider various other factors which matters much more like fabrication process, transistor count, core config, clock speed, processing power, etc. understandably, that's confusing, so that's why it's much more recommended to look at benchmark reviews and compare from there.
vram at most helps with frame buffering. in a gist, between a 2gb & 4gb 760, the 2gb variant will likely see a larger fall in performance (fps) compared to the 4gb card when you bump up the resolution from 1080p to 1440p, or 3x 1080p or higher, provided the other specs don't hold it back, like memory bandwidth. useful for people who already have a 1440p/1660p or above monitor, or need a higher res monitor for their primary task, like graphic designers who do a side of gaming sometime. for the mainstream gamers, though, 2GB GTX760 for 1080p, or a GTX770 for much better performance be it for 1080p or a higher res.