single 1080p monitor, you will always prefer 1 single more powerful GPU.
With the GTX 770, if you get the 2GB model now you will regret it as in SLI the vram is not added but mirrored, so if you get the 2GB card, you will only have 2GB between them.
2+2=2
2+4=2
4+4=4
If you go SLI, you want the 4GB GTX 770. For you I would strongly suggest the GTX 780Ti.