If that is the case you are going to need an external power supply like an optical bay one because the GTX 680 use 2 6pin connectors and using power splitters is to risky. There is a sticky on this forums that explains which PC GPUs you can use, the GTX 680 is one of them. But if you are going to use a PC GPU and a MAC GPU at the same time there are going to be issues, so basically they have to be both PC GPUs or MAC GPUs.