Well Wattage is almost meaningless, sadly.
What you need to decide is this :
a) Does the PSU have enough PCIE 6-pin power connections to work?
b) Does the PSU have enough theoretical +12V rail amps for the PCIE connectors, motherboard, and so on?
c) Is the PSU high quality and reliable at delivering 90% of its rated output over long term use? Some "500W" supplies blow up if you ask them to deliver 300W, so there's a big difference between quality engineered supplies and bad ones.
The answer is that you'll almost certainly have a whole-system WATTAGE of under 450W, probably even under 330W for the whole system under GPU / CPU load.
A good quality 500W PSU with 24 pin M/B power (assuming you have that), a 4 pin M/B aux. power output, two 6-pin PCIE power connectors, and plenty of continuous amps on +12V should work fine. A lot of low quality 500W PSUs would NOT work fine, though, since they'd lack enough +12V amps or good stability or good cooling or whatever even though your actual power needs are maybe 450W or less.
Thus to be "on the safe side" a good quality 520, 600, 620, 650 PSU might be more likely to work. The real factor isn't the wattage, it is the quality of the supply and ensuring that you're asking maybe 70% of what it can (in marketing speak) actually deliver just because you may not be able to trust it to give 80%, 90%, 100%.