500W indicates older models, 550W is mostly newer models. While few and far between, there are models from Corsair, Seasonic, and EVGA that I would not recommend. Check back here before you buy anything.
The point of the low-powered RX 550 is that you don't have to upgrade your power supply. Might as well go with the RX 560, which again is meant to not have to upgrade your power supply.
That said, if you care about your system you should upgrade to a decent quality power supply.