I can't tell you for shure because I dont have 680, but I can fly over London,
with 2x weaker 560 Ti 1 GB with only few settings put on medium (building details, land details and forest).
If money is not a problem and you decided to buy 680 there are models with 4 GB, but if you going to play on 1980 or lets say later on 2560 resolution,
I don't think you need it.
Also I think that gtx670 is best buy in that segment because it lags only few percent behind 680.