I'm looking to power this kit:
Intel Core i5 6600K Skylake CPU (O/C to 4.6GHz)
Asus Z170-K motherboard
Be Quiet! Dark Rock 3 cooler
16GB Corsair Vengeance 3000MHz DDR4 RAM
MSI Radeon R9 390 8GB graphics card
1 x SSD (probably a 256GB SSD)
1 x HDD (probably a 1TB HDD)
I imagine a decent 650W would be plenty? Just to reiterate CPU would be overclocked.