
Assuming your projects fit in 1TB of space, your storage is fine. I've run Fusion on 16x SSD RAID 0 arrays, and it wasn't much better than running off just 3x array, so I'd leave that alone until you need to add capacity.
The second Titan won't do a thing. Fusion only uses one GPU 99.99% of the time. You have to have to be running an OpenGL render AND an OpenCL render concurrently for it to use the second GPU, and with so few OpenCL and OpenGL tools, that's actually very rare.
Doubling your memory, however, is a surefire win for anything other than transcoding. That would be my strong recommendation.