If I am intending to sli 2 gtx 670, (i am only using a single monitor now and will only have one in the future 27inch) should i go for the 4g cards so that i can use the highest aa and also so the cards will be future proof for games and CAD software?
Ditto, with graphics cards you can only future proof until the next generation cards before peformance drop off is high, sure games and all will run fine along with apps but a lower end card of the newer generation will outperform it.
A 2GB version is more than enough for a single 27 inch monitor at 1080p; 4GB is excessive. Even with two GTX 670 in SLI you will still only have 2GB of VRAM still. Two GTX 670 is excessive too, but not a bad idea.
In borderlands 2, with all settings in both the Nvidia Control Panel and in the game settings completely set to max possible settings, I average 90-125 FPS at 1080p. Later when GTX 670s drop in price I will go SLI, but for now one is overkill for almost every game at 1080p. You will probably want to use adaptive v-synch, because screen tearing too unless you get a 120hz monitor.