Your motherboard is not SLI certified. It is Crossfire certified. Afraid it can't do SLI? Speak to your insurance agent.
Crysis 3 prefers Nvidia while Battlefield 4 prefers AMD
Both GTX 770 and R9 280x are capable of running Ultra on both games.
Assuming your buying in the US both cards are priced competitively. But I'd pick the GTX 770 as it offers more features that aid video encoding, recording, and comes with free games.
If your not in the US, tell us what country, how much you willing to spend (in your local currency), and link us to websites you can buy from.