None. The Parallel Computing Toolbox can execute on CPU threads or cores.
Graphics cards would only be a consideration if you were wanting to use GPU computing. If that is what you are interested in, then "MATLAB supports NVIDIA® CUDA™-enabled GPUs with compute capability version 1.3 or higher". This is a significant restriction: the majority of the NVIDIA graphics cards only support CUDA 1.2. There is a table of support here
