If you manage to crossfire without having micro-stutter problems, a second 7850 would probably be the biggest "bang for your buck."
I have two (overclocked, non-ti) 560's in SLI and currently get between 80 and 90 FPS at 1680 X 1050.
Two 7850's should do at least as well. (Probably a bit better)
Now, the questions are: At what resolution will you be gaming? (higher resolution will be lower FPS), and how much tougher will Crysis 3 be?
Generally, a single card solution is better than two cards, but I have someone interested in buying my SLI cards, but it seems that a single 670 will actually be a downgrade for me in FPS on that game. (Although a quieter GPU experience would be nice)
http://www.tomshardware.com/reviews/geforce-gtx-670-review,3200-6.html
So crossfire setup will be cheaper for you and probably produce slightly higher FPS than a single 670.
Crossfire runs the risk of micro-stutter.
And extra noise is pretty much a given, not a risk.
If you can buy a second 7850 from a place with a good return policy, keep the packaging nice and neat and test a crossfire setup, I would do that.
If you get micro-stutter or decide the extra noise is too much, just return the extra card and go with a single-card solution.