From what I've read, the GPU's are really the heart of your mining rig, and the rest of your hardware is just there to get your mining software hooked up to your GPU's. The only real consideration in selecting your other hardware is to make sure you have an adequate power supply for your computing needs (bronze level certification for clean power recommended), to make sure that your motherboard has enough pci-e slots for all your GPU's, and to make sure that your case is adequately ventilated so your cards don't overheat.