He is using a 120hz monitor. That means he is either doing 3D gaming or aiming to get 120 frames rather than 60. For a standard 60hz 1080p monitor, A single 770 would probably be fine, as would a 780.
A single 770 or 780 will not get him 120 frames in the latest games or 60 frames in 3D. If that is your goal OP, I would probably go with two 770's. But keep in mind what others have said about dual card noise, scaling issues, and heat.