Should I crossfire 2 Radeon 7970 3GB cards or should I just buy 1 NVIDIA GeForce GTX 680 4GB. I head Crossfiring can have problems or not increase performance on some games and even make them unplayable sometimes
Then you should most probably stay with one. If you do get triple 1080p screens for gaming and you found out that you want more performance, that's when you should actually get another 7970 (Or maybe get the newer gen cards if they're released by then).
the difference in performance between the Radeon HD 7970 and GTX 680 is minimal so upgrading to a GTX 680 will not gain you anything useful. See this for comparison: Radeon HD 7970 CF vs. GTX 680 SLI.
If you plan to run 3 screens at 5760X1080 then you should get a second Radeon HD 7970 to crossfire with your current one. Otherwise you would have to buy 2 GTX 680's in order to get similar performance.
If you plan on staying on single monitor for gaming 1x 7970 is plenty
If you plan on gaming on triple (you cannot game on 2 monitors, has to be 3 min)
you will have to lower graphic settings on most games, else u will need a 2nd 7970 to keep the detail settings maxed out