At 1080p I found 2GB adequate enough when I had my SLI 680's. Some newer games I'd have to turn down a setting or two, but for the most part they handled pretty well. This was not the case once I upgraded to a 1440p monitor and it's the sole reason I upgraded to a 980Ti last month. It was my intention to hold off until Pascal, but I just couldn't hold off that long.
Same here. My 670's did well for about 18 months but then I had to start turning details down at 1080p. With todays games, even at 1080p, forget it. Details would have to go way down on some of these new games for a 2GB card.
In a year I expect 1440p to be 8gb territory for some games and 1080p will be 4GB+ territory with 2 and 3gb cards totally choking out. By the end of the year, there will be games that blow past 6GB at 1440p. GTAV almost does it right now. 4K will be for dual 12GB+ cards in a year.
Vram is the hot new thing. Devs are using it up like absolute crazy, just because its there. So, better buy a new card every 18 months or get left in the dust. Those devs need their Vram (they don't actually need it but they want it for some silly reason). I blame consoles.