Depends on what you want to do or play. Even when I only had a single 780 it could handle most games on 5760 x 1080. You may need to keep watch on the AA as that is what really starts to bog your GPU down. It is much better now that I have two 780s but most games were more than playable on either max or very close to max.
If you have the money and the PSU then definitely go SLI. 2 x 980s will crush anything you want to play at 5760 x 1080. You could also pick up one and see how you like the performance. If it not to your liking then you can always pick up another one.