Not too sure mate, I use 2 of the same cards in my PC and tbh I dont really see any difference with one in there (never play games) there set up in crossfire mode running dual monitors (though I've now got a different monitor all together so its only one now.

I know a guy who mixed his cards and it worked, I didnt think it would but it did, think it was an gforce and an ATI, not too sure what, if any problems there would be but I would try to match therm really.