I would probably go the better card route, sell your old card and get a GTX 780 if you want the Nvidia route, or wait for one of the new Radeons when they come this fall.
I currently have a gtx 570 2.5gb sli rig and while its awesome performance wise for triple wide at decent fps, there are drawbacks such as microstutter where my framerate can be capped at 30 and once it goes to 29 itll feel like its at 20 fps lol
getting a single badass GPU is better than a dual setup from what i learned lol
this is my advice, but there are plenty of more knowledgeable folks on this board that can probably prove me wrong
happy hunting!