Yeah I know what it says, and when you experience low frame rates you know the things to avoid. There's a difference between 'reading' a review and actually experiencing it first hand - repeatably and annoyingly.
I have no desire to repeat problems and I know on 5900x1080 I will run into vRAM bottlenecks with 2GB -- no doubts.
Therefore, I can tell the OP all kinds of flowery & rose colored BS or warn them ahead of time, and I choose the later. I'd be running 3-WAY 2GB GTX 680's now if I wasn't convinced to wait for the 4GB versions.
The OP has a choice, and the 'good thing' about SLI is you can add a 2nd, 3rd or even a 4th GPU *IF* your MOBO supports it. Personally, in this setup I'd either go for the SB-E/LGA 2011 (e.g. ASUS R4E) or IB/LGA 1155 (e.g. ASUS P8Z77 WS). The day a decent EVGA/ASUS 4GB GTX comes out I'm personally adding '3 to cart', otherwise you 'might' be stuck waiting...