Dual 290 kills 770... and 780 Ti in turn. I admit, a single 780 Ti can get OK frame rates on most games at 4K on the highest settings, but you would definitely require two for a comfortable 60 FPS, and then some more OC performance to keep that up across all games.
For the price of two 780 Tis you could grab three 290s with cash to spare though, so if you really are after that 4K 60 FPS or 1440p 120 FPS, I would not go with the 780 Ti route unless you can happily afford 2 with the intention of getting 3.
Also note that Hawaii chips seem to scale extremely well in crossfire (from what I've seen - and there are always exceptions).
However, that is all a different discussion.
Also known as RipperRopper3000