Im planing to set up a small energy efficient mining rig, im planing to use a low power CPU/MOBO bundle, this is my favorite: Biostar A68I-350 (with 1 DIMM DDR3 4Gb)
It takes only 18W TDP and has one PCI-E 16x for a HD7950. The total Wattage is (considering the usage of a 80+ GOLD PSU) lower than 300< Watt. The KHash/s rate will be 640-650.
How much can the CPU be the cause of a BottleNeck?