If you have money to spare, get a Titan. If you have lots of money to spare, get sli titans. Alternatively, if you have no beef with amd, get an r9 290x or two as their 4gb is the best you are going to get outside a Titan.
What? The Titan is almost a useless card. The 780ti is cheaper and faster.
If you are going to be doing multi-monitors, then yes, get a 6gb card.