Possible Upgrades

I posted this on the mobo, cpu, SSD section but since I had video card questions I thought I would repeat it here. Thank you.

I have a PC I built to be my main computer of which FSX was a main consideration.  I have decided to move to a MacBook Pro Retina to be my main computer to do all my surfing, photos, music, videos, etc.  As such, I am going to dedicate my PC as a flight sim and hopefully, build a general aviation cockpit.  Please take a look at my signature for current details.  

First, I thought I would upgrade my current SSD to a 256GB.  My 120GB is full.  I currently run 2 of the 560TI video cards in SLI so I can use 3 monitors.  NVidia now has single card solutions like the GTX 670.  As a consideration, I hope to add additional monitors in my cockpit for instruments.  I am not sure if a new video card would be cost effective but I throught I would mention it for discussion.  Please offer any other suggestions.  Once I have the computer just right and running perfect with my current accessories I hope to begin the cockpit.  I hope to be able to have the outside visuals only on the monitors and my instrument panel/radios,etc as separate hardware.  Thank you for your help.