These are the official recommended power requirements based on your graphics card:
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode).
This applies to the entire system not just your graphics card.
As you can see, a 650W unit will be above the recommended for your card even if you opt to Crossfire in the future. If you do want to have the extra headroom to be safe for future upgrades then the PSU sportsfanboy mentioned is a very solid unit, as is the one Corsair you pointed to. It may also be worth mentioning that PSUs are most efficient when they're running at less than their full load (usually about 50-60%). That being said, the 650W unit would do the job fine if you're on a budget. Hope this helps.