You need an SLI cable, the mainboard must support SLI and it should not be a cheap mainboard. The power cable comes from the power supply you need a 6 pin plug and an 8 pin plug. Since you want to use two graphics cards in SLI, I would recommend at least an 850 Watt power supply, if not higher.
Sorry for the spelling I am still a student xD
Should be enough power except for the graphics memory. Unfortunately, if you run two graphics cards in SLI, the memory does not double. That's why I'd better take a GTX 980Ti. Or wait for the new generation, which is coming soon. I myself have an Asus GTX980Ti OC and play 3840x2160 with approx. 30B/s with high settings!
The 850W definitely refers to the entire system, but is probably significantly oversized. For comparison, a 750W power supply is recommended for an R9 390 (max. power consumption 375W). However, I only have a 550W power supply, which is completely sufficient. Therefore, I recommend searching Google for "power supply calculator". There you enter the components and in the end you know what kind of power supply is sufficient.