Ok here i belive (im not 100% sure) that people are mistaking a few things.
On 1 side we have the Resolution, then we have the AA technology used and finally we have the Textures.
All those will define how much ram your video card needs in NON-Crossfire/SLI.
In other words, SLI/Crossfire does NOT add up the ram.
So for 1900x1200 or less, assuming you use todays AA systems (FXAA or MLAA), 1 GB is hangin in there for heavy textured games. It means its close to its limit.
HOWEVER: The games that will run slow (lets say Metro 2033, batlefield 3, skyrim, ETC) will do so before your GPu needs more speed rather than more ram.
In other words: IF an SLI configuration has a decent boost in a game, that game does not have enought textures at the current resolution to require more RAM.
A clear example of this can be found a hardwareheave.com or hardocp.com
Those sites test games in eyefinity many times and you see there what is and what is not bottlenecked by ram.
I hope this will give you a general idea.
As a final note, if you have heard some of the stuff surrounding the game RAGE from ID, that is supposed to have up to 8k independant textures, it runs really really smooth on my 560 Ti with 1900x1080, but its clear the engine is good, but weak.