ok, I dont like work in labs at Nvidia or anything but here's my 2 cents:
video cards used to have ram for nothing more than a frame buffer which meant the higher the resolution (X pixels times Y pixels) times the higher the color depth (16 bit color, 24 bit, 32 bit etc) dictated how much ram you needed. then along came 3d cards which added ram primarilly to hold textures.
today, video ram is used for this same function, some of the ram is for frame buffers but if you calculate it out even at the max resolution today - (1900x1080 = 2,052,000 pixels X 32 bits = 65,664,000 bits / 8 = 8.208 megabytes of ram! that's it! (yes there's double buffering and triple but still a drop in the bucket).
so 99% of the ram is left over for textures and rightly so. why? cuz games need many many textures to make 1 on-screen scene that's why. you have grass, tree bark, leaves, sky, facial, clothing, etc etc. and each one can be 4096x4096 or more. that's where all the ram goes.
now realize that when you run SLI, each card only has to render 1/2 the screen, but it still needs the full "pallet" of textures to render its scene with. so each card has to get all the textures loaded into its ram that the entire scene needs anyway.
in other words - SLI really has nothing to do with ram.
higher resolution textures, scenes with farther visible distances, having headroom to load up-comming textures for the area you're moving to. these are the driving factors to how ram gets used and why more is important in graphics.
I've heard "benchmarks don't show any benefit to more ram", well DUH! benchmarks are about speed in processing, the card still processes the same so its not going to show in a benchmark. that's like comparing horsepower with (2) 400 hp cars that have different size gas tanks! of course they have the same muscle but one goes farther than the other!
how much you need is still a tough answer. but if you're looking at a card that has X amount of ram and THE SAME VERSION card is available with double the ram for an extra 10% higher cost. I say its worth it.