Just something I've gleaned from the net, but from what I've read, it goes like this:
128bit memory interface = 1GB VRAM
256bit memory interface = 2GB VRAM
384bit memory interface - 3GB VRAM
512bit memory interface = 4GB VRAM
Basically, the memory interface would be a limiting factor, like a GTX760 4GB, most throughput that can be used is about 2GB to 2.5GB tops, so having 4GB on this card is pointless (also assume the GTX760 has enough GPU grunt in the first place) unless you go SLi where, someone correct me if I'm wrong, the memory interface can be stacked, unlike VRAM. For Watch Dogs, guru3d recommends High quality setting with FXAA for 1080P gaming, turning up AA to 2x/4xMSAA will eat up VRAM.
Watch Dogs VGA graphics performance benchmark review - Article - Guide - Review
Again, let me emphasize I am no expert, just something I'd picked up while surfing.