According to RHTX, minimum recommended is 550w. That's also roughly good for a gtx970/980. General consensus has a 390 at 650w,same as a gtx980ti. Yes the 380 will run just fine on 500w (please find me a 'good' quality 500w) but op did mention possibly upgrading in the future. Not knowing what power will be needed in the future and just guesstimating next gen gpus will be roughly equivalent, Op could save having to upgrade the psu if later deciding to go with a stronger gpu.
Oh, and as it was explained at length to me, those excellent leadex platforms are failing ATX standards, that the Seasonic are passing, just saying.