These are the official recommended power requirements for your 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 your graphics card.
As far as the size of your PSU goes, you will find that most PSUs for desktop systems adhere to the ATX form factor meaning they all conform to one size and standard.
Something like the Corsair 650TX would probably do you nicely.http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005
So as you said if you do plan on using a crossfire setup in the future then you would need at least a 600W PSU. Corsair, Seasonic and the newer Antec models are the way to go.