r9 270x uses 142 W in bitmining (99-100% load), your other system components usually use ~200 W more, which means your 500W PSU should be enough for it.
But the problem is, you didn't tell us your PSU manufacturer! If it's a generic 500W PSU I wouldn't recommend you using it.
500-550W PSU models from Corsair / Antec / SeaSonic are quality and will bring you great performance and reliability for years of usage.