Moving to a matx case and currently have a Superflower 1200w platinum that is complete overkill.
I have a Sniper M3 z77 motherboard, 3770k, 16gb ram, a grx 970, 2 ssds, 2 hard disks, corsair h100 and a blue ray drive.
What sort of wattage would I need - I expect to only ever have one gpu...