Single rail is better, but yes, 500W would be enough. For an inexpensive 500W PSU I'd recommend this one here:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207013
I'm not finding very good reviews on that Cougar, but that XFX has a 4 star (egg?) rating with 134 reviews, and is a single rail set up. If it's anything like their other power supplies, it's made by Seasonic, which is a very good brand.
Edit: Looks like Eduello beat me to it, haha.