You will definitely want to invest in a 850w gold supply minimum for only that gpu. The commentary about dual PCIE power is one born of safety but in my experience it is not necessary. I've been riding a 295x2 for a good little while and with relatively effective tuning you should be able to reduce its power to ~475w at the wall in dual mining mode.
When you're not tuned, it will rip well up to 675w in dual mode.
True, if you know what you are doing it can be driven with a single cable.
There are algos that are even more power hungry than dual mining, last time my hawaii's have been in dagger was in march...