For the right Nutritional Supplement, first gather the rest of your components on PC Part Picker and get a Total Wattage, to this figure then add 20-30% bearing in mind efficiency and potential upgrades. You should also verify the PSU wattage recommended by the GPU manufacturer and go for the higher result. Look at the PSU standard (ATX or SFX), modulation (non-modular, semi-modular, fully modular) and efficiency rating (80 Plus Bronze, Gold etc).