I am unsure why you would want to upgrade to 2 monitors bc if you plan to game with 2 monitors, it won't work well. You need 3 monitors and its worth it for 3 monitors. Secondly, yes an upgrade to a gtx 770 is worth it. I would hope if you are going to get a third monitor in the future so I would recommend the gtx 770 4gb. I wouldn't get the gtx 770 2gb because you would need at least 1gb on each monitor to play with ease on high settings for today's games.
Decision: get 3 monitors buy a gtx 770 4gb, if you will never upgrade to 3 monitors then a gtx 770 2gb will do just fine.