Usually, the site you buy it from will have specs. I know for sure Newegg does this but it should say the wattage required. Get about 100W above that. My R9 270 needed a 500W. I got a 500W :|
EDIT: Looks like your card, under brand EVGA, requires a 400W. It would likely go under stress like mine does sometimes. I would get a 500 or at least a 450