I don't think this belongs in custom hardware but just hardware as custom hardware is not GPUs and CPUs but FGPAs and ASICs
Ah my bad, i thought Custom Hardware would involve home-made rigs.
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.
Alright, thanks.