These are the official minimum power recommendations for your graphics card:
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
This applies to the whole system not just the card.
As long as you're not planning on Crossfire in the future then a good 550W PSU would be adequate. Recommended brands are Corsair, Seasonic, Antec etc. Avoid anything cheap.
Hope this helps!