What is the min - avg Wattage PSU to get?
Core i7 c/w itx Board / Z87 Chipset
8 GeeBees of ddr3-2400 RAM
R9 280OC (Tahiti Pro, Radeon HD 7950/4950 OEM)
56gb OS, 5TB of spinners (3-HDDs)
Minimal gaming, no overclocking
you're gpu uses around 300 watts when gaming and 84 watt cpu so while technically a 500 watt psu should be enough i would use 550-600 min.
pretty safe with a 650.