Both of these suggestions are very helpful. I agree with Alexander, perhaps it would make more sense to get 4 iMacs with higher specs and I could use the left over money to upgrade my Mac Pro, which is currently 2.8mhz Quad-Core Xeon, 12GB RAM, non-CUDA GPU. I could add more RAM, an SSD and CUDA-GPU to it and be just fine.
Obviously, SSDs are ideal for cache so should I upgrade to 512GB SSD internal (a $300 upgrade) and keep cache/scratch on the OS disk… OR keep the 1TB Fusion HD internal to use as media storage (and hopefully SSD application speed) and purchase an external SSD for cache, but what size would be appropriate?
Thank you so much for your help.