I Would Look at the Intel Family and the NVidia 8xxx or Higher Series cards.
Mainly because the NVidia Cards with CUDA click here can be used to offload Accelerated interconversion of video file formats. You won't be 'using' the Graphics cards GPU when coding and recoding video so it (the GPU) can be used by the system to help speed up the conversions.
And if you go with NVidia then the best pairing for that is the Intel CPU not the AMD. However, cost may prohibit you there are both CPU and GPU are more expensive on this pairing.