I would worry about a 500w psu unless its a very good quality unit,my cx500 should have been able to power up a system that peaks at 300w with my new gtx770 which pulls comparable wattage to the 7970 but what happened is my system locking up and the funky smell of a stressed out psu.
Had to order a tx650,hook up my spare 17'' 1280x1024,drop tons of settings and enable vsync as a temporary patch so i don't pop my psu with high gpu loads and usage......hopefully my gtx650 arrives by wednesday.