Tricky, if you can possibly afford it I would suggest you go ahead and grab a 6GB Titan and do some testing with your typical files. Render them to your noise level satisfaction and do some simple math regarding what the times would be with more GPUs. Unless you are comparing Fermis to Keplers, they seem to scale in a very linear manner. Further, you will be able to see how much RAM your typical file will take to render on the GPU which will give you more valuable information regarding rendering with RT/GPU in your future.
Even if you do not decide to go with rendering on the GPU for final work right now, you will still have an excellent board for quickly designing materials and lighting that you can add to the GPU fray in the future, and you only spent around 1000$US.
Alternately, I do have a friend who just installed 2x 6GB Titans in his workstation. If you could ftp me one of your typical files perhaps we could do some test renders for you and let you know render times per noise level, RAM usage, etc. We would have to make sure the file is GPU compatible of course so if we do this, we could talk a bit about how we could go about doing that.
Best,
-Alan