CrossfireX / SLI - the Corsair HX650 doesn't care.
As long as there is enough +12V power for your cards and the rest of the system too.
It has 4 x 6+2-Pin PCI-e power leads for the graphics cards and 52Amps of +12V current.
What cards are you thinking about SLI'ing?
Dual SLI nVidia GTX 550