For Ati the have to be the same core. They can be different manufacturers, memory sizes or clocked differently (each card runs at it own speed)
For example:
HD3870 + HD3850
HD4870 + HD4850
http://game.amd.com/us-en/content/image ... July08.jpg
For Nvidia they can be different manufacturers, different clocks(card clocked to lower speed) and different memory size. I suspect that the number of shaders must be matched as well, which screws up things on a pretty collosal scale on the Nivida side as they have cards with the same names that are different, and cards with different names that are the same (9800GT = 8800GT)