Just a note, you can always use an (additional) separate power supply for the GPU card. Cross 13 / 14 to keep it on.
http://makezine.com/projects/computer-power-supply-to-bench-power-supply-adapter/
Using a “GPU external power”, means a low specification board and PSU can be used for a mining PC.
A low power “used” PSU, just to power the GPU. I extracted a few from some old PCs from Freecycle… Neoscrypt can not drain the GPU melting currents of high Intensity possible with Scrypt, because of the memory requirement, so even a old 350 Watt worked fine.
Use a power plug / hall effect clamp (from power company), to accurately assess how much the PC and GPU power are taking.