You can have them in CF only problem is you have to either underclock the gigabyte OC one or OC the Sapphire card. They just have to run the same core and memory speeds. Besides that brand doesn't matter at all only model. AMD also lets you run two cards of the same series together, for example any 68xx card so if you really wanted to you can CF a 6850 and 6870.
Ok so im a bit confused, the guy above said that hes running crossfired differemt bramds but different speeds without problems