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No I think you should get a 750 W power supply with a 80+ bronze certfication as that would ensure that your system gets a sufficient amount of power.
I run a 290x with a Corsair HX650 (650 watt) Gold power supply. It runs great. Do not buy a cheap power supply. I would avoid a bronze power supply and try to get at least silver, but aim for gold.
The main problem is that your Power Supply is pretty old, its hard to predict its current state and performance.
The wattage itself is not a good guideline for choosing a suited power supply in the first place. It is most important to get a power supply from a descant brand and series such as Superflower, Seasonic, Corsair (not the vs series) and cooler master. Make sure that the model is at least 80+ bronze certified and has the appropriate PEG connectors. Recently I tested the Antec VPF 450w which ran my Overclocked 2500k and r9 290 absolutely fine without getting hot or noisy.