A good 850w will be fine, unless you are heavily overclocking the system and or using a 220w space heater as a cpu. I have an I7 3820 clocked at 4.3Ghz with a Gigabyte Xtreme 980ti running at 1500/7400, 4 hdds, 1 ssd, a water cooler and under full load(100% cpu and 100% gpu with furmark) and Corsairlink shows a max pull of 500w.