I don't think this belongs in custom hardware but just hardware as custom hardware is not GPUs and CPUs but FGPAs and ASICs

The CPU needs to feed the GPU, handle comms (to/from pools), etc.. so to an extent - yes.  Pretty marginal, though - any modern CPU is 'good enough' if not overkill.  If you're going to mine altcoins, you might also check out whether there's a CPU portion involved, still.. that would have a greater impact, but not nearly as much as your choice of GPU.