A 550w power supply would be plenty to run that system, but I would say 600-650 would be around optimal. If you want to Crossfire in the future, I would get at least a 750w power supply, with 850w being close to optimal. But the key to choosing a power supply is to make sure you get a good quality brand/model, not just look at the wattage it can supply (or says it can supply)