What everyone else said...
Basically the configurations are different between generations. For ATI, the first two numbers have to be the same to have similar configurations. NVidia has to be exactly the same, and it's called SLI, not Crossfire. You can't combine NVidia with ATI. You can combine different manufacturers though, like XFX 5870 with Sapphire 5870.
XI - Draiden 75DRG/75COR (Sylph, Retired 08)