It seems like most people on here are running multi-GPU (3 or more vid cards) rigs... am I the anomaly in that I'm running a single card at the moment??

I'm running XMR STAK with a 980 GTX and i7 4790K getting 815-825 H/S combined (CPU and GPU)... is that decent?  I'm thinking of adding an RX 580 into the mix, but I doubt my Corsair Gold 750W could handle it?