You are correct - that PSU is more than enough. It could handle 7850s in CF without a problem.
550w and 7850 cf,hhhmmm are you sure?
600W power supply (or greater) with two 75W 6-pin PCI Express power connectors recommended for AMD CrossFire™ technology.
i am a big xfx fan but i think a 600~ will leave enough room for overclocking and crossfire.
but i have a solution too,silverstone strider 600w for $69-http://www.newegg.com/Product/Product.aspx?Item=N82E16817256071
silverstone products are one of the finest and i have the same opinion for this power supply as it is highly rated.
@islamthebest2,enjoy your build.