they probably meant 3 1080 screens. Yours are pretty low Res and I assume the 1680x1050 screen is going to end up running at 1680x800.
Look at the 2560x1600 benchmarks.
Your Res would be 17% lower. I doubt you'd be memory limited. or need SLI.
If you SLI a 4GB with a 2GB card the 4GB card will only use 2GB so its a waste to do that. memory DOES NOT add in SLI. If you plan to upgrade the monitors to multiple high Res displays you might want to consider a 4GB though