GTX or Tesla input sought to build a new sytstem


currently I programm in C# using Cudafy and run my computaions on a GTX560 ti. I need to do a lot of double precison calaculations.
I’d like to build a new system specifically designed to run calculation on GPUs. I wonder whether I should get a system with multiple e.g. GTX cards or with one Tesla C2050 or 2070 card. What kind of mathorboard and CPU would you recommend?