the upgrade to 2gig VRAM is beneficial only in some of the higher-end games, where 1 gig is simply not enough. the witcher, for example, used every bit of 1 gig at max settings on my laptop. The new Medal of Honor uses about 1.3 gigs.
the increase in video memory doesnt increase performance, it's the clock speeds (core/shaders/memory), memory type(gddr3/5) memory architecture bandwidth (64 bit/128 bit, 256 bit) and a number of other things.
SLI is very beneficial in gaming in that it basically takes the load and splits in half amongst the two cards. The slave unit will render it's part of the load, send it back to the master unit, which will then combine the two and send the image frame on its way. SLI can render images in several way- either by splitting each frame in half so the two cards are rendering a portion of the very same frame. it could also alternate between frames, here one card will render a frame, then the other will render a frame...alternating back and forth. It can also be used in a method where the major rendering workload is put onto one card, and the other card basically works to enhance the original image. the third method generally produces a much prettier picture, since it opens up further rendering options by utilizing SLI that you cant get any other way(such as 32x anti-aliasing, and 64x in quad-sli setups!). In the third setup, it works in the sense that the first card is enhancing the original image with the preset settings, then the second card will take the image and further enhance it as if it wasn't enhanced in the first place.
