For my first build, I plan on using an i5-6600K and an R9 390, which paired with all of my other peripherals consumes 475W without any overclock, which I plan to. I also hope to eventually go CrossFire, and was wondering what wattage of a PSU I should get if I plan to overclock both graphics cards and the CPU. Without overclocking, the setup would consume 750W of power. What wattage and efficiency power supply should I go for?