I have a similar setup with 770 4gb in SLI. If you are only running 1 monitor I would say no, it's not worth the 300 extra dollars. If you plan on a surround setup in the future it may pay off to go SLI.
It depends, some games are optimized for SLI, and some will give you a lower fps than a single card. Your PSU and CPU should be able to cope with SLI, and make sure you have enough fans.
I have a similar setup with 770 4gb in SLI. If you are only running 1 monitor I would say no, it's not worth the 300 extra dollars. If you plan on a surround setup in the future it may pay off to go SLI.