Consider these if you prefer single card solutions:
Nvidia: 770 4GB, 780, 780 Ti
AMD: 280X, 290, 290X
Otherwise, getting another 560 Ti won't be a bad option.
I personally recommend the 780. It has adequate amount of VRAM at 3GB. It won't break the bank that horribly priced at $499 (varies). The 290X is pretty much made pointless by the 290 which is essentially just the 290X but with lower clock speeds at a much cheaper price but I'm just giving you more options.