Quoting Guru3D.com (from your link above):
Much like the GM200 Maxwell GPU harbored in Titan X, this GTX 980 Ti is rated as having a 250 Watt TDP, our measurements back that up, bear in mind that this is a peak maximum value under full stress. At this performance level you are looking at a card that consumes roughly 450 Watts for the PC in total, that is okay. We think a ~650 Watts PSU would be sufficient and, if you go with 2-way SLI, an 800~900 Watts power supply is recommended. Remember when purchasing a PSU, aim to double up in Wattage as your PSU is most efficient when it is under 50% load. Here again keep in mind we measure peak power consumption, the average power consumption is a good notch lower depending on GPU utilization. Also, if you plan to overclock the CPU/memory and / or GPU with added voltage, please do purchase a power supply with enough reserve. People often underestimate it, but if you tweak all three aforementioned variables, you can easily add 200 Watts to your peak power consumption budget.
So roughly 450 watts for the base system and the GTX 980Ti without overclocking, and another 200 watts to cover overclocking. We have reached 650 watt potential used, and just to be safe and run the power supply at less than its maximum load, I would go with a 800w to 900w unit. I would also make sure I selected a high quality power supply. You don't want cheap stuff at these power levels.