leekulet :
Can a seasonic 620w psu handle the strix r9 390?
More than sufficient - no need to upgrade. The key spec is the +12v rail amps. A system with
one R9 390 requires 28 +12v amps, the Seasonic 620(S12 or M12) has 48 +12v amps.
You can OC(tho your CPU won't OC) if you wish.