Re: what PSU wattage will be needed to crossfire two ATI HD5770's
A 600W PSU by a decent manufacturer would be more than enough. The 5770s are very frugal.
I had an HD 4870 X2 running off a 600W PSU and that would suck up more power than HD 5770 CFX.
Remember that manufacturers also have to keep in mind the large amount of crap PSUs when they give their recommendations, so they are always on the safe side.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.