Does anyone know the difference between 2 770's SLI and 1 780, FPS wise? On Crysis 3 for example (using a BenQ XL2720T 120Hz monitor)
Any benchmarks would help. I've found other threads about this, but no benchmarks or demo's.
Get one 770 4gb for your rig, sell it in a couple of years and upgrade to 960-970. The high end GPUs are not really worth it. They have their place but... I find value more in the upper high end. Single cards run cooler, use less electricity. A single 770 4gb should be enough for 95% of the games out there on ultra right now.
You will win benchmarks with sli GTX770 compared to a single GTX780.
In the process, you will have paid more.
You will also be exposed to dual card inconsistencies, noise and heat.
With your 1080P monitor, I see little reason not to use a single GTX780.
If, in the future you get into triple monitor gaming or get a 4k monitor, then all bets are off.
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.