I have an XFX 5770 and the opportunity of getting a Sapphire 5770 at no charge....
Is it possible to crossfire 2 5770's from different manufacturers?
ASUS P6X58D / i7 930 / 6GB Corsair RAM (3x2GB) / win 64 ... if this makes a difference
So-long as your graphics cards have the same speeds they will work fine, If one is overclocked however it's speed will be reduced to match the lesser card in the configuration.
All you should need to do is install and put in the correct power plugs, put the Xfire bridge on and then make sure all your settings are correct in CCC and then viola you should be finished.