The R9 290 has been known to consume, at absolute maximum load, 372 Watts of power. It averages at more like 250 Watts, however.
Given that it's quite a power-hungry piece of equipment the more Wattage you can throw at it, the better, however, a 600w PSU will be more than enough to allow the card at around 80%-90%.
Just make sure your PSU has the correct PCIe power connectors to power the card.