I want to output via SDI, do I need a NVIDIA GPU anyway?
Yes, CasparCG needs the GPU for internal processing and not for output, so you need a OpenGL 3.0 GPU to run CasparCG.
SVT recommend to use a NVIDIA GPU of the Quadro range. They are known to be very stable.
To choose the right GPU for your CasparCG hardware, keep in mind, that OpenGL processing power is more important than the number of gpu cores or the cuda power.