From what I've gleemed, visiontek has a lifetime warranty and is available at major retailers like Best Buy.
From what I've heard through the grapevine.....stay away from Sapphire!
Your powersupply sounds fine but if you're planning on Crossfiring your cards...you're gonna need a 750watt or more.
VisionTek does have a lifetime warranty
As for PSU requirements http://www.newegg.com/Product/Product.aspx?Item=N82E16814129113
crossfire setup requires minimum 600 watt PSU.