AMD Crossfire compatibility

[abizer14]

I have a Sapphire AMD HD 7770 graphics card and was planning on getting a new card.I have 2x PCIex16 slot on my mobo so was wondering if I could crossfire the 7870(which I might buy) with my current card. As they both are Sapphire AMD and from the 7xxx family series they should work according to me but I still need a confirmation as I am not sure. Secondly, in crossfire will the performance of the 7870 come down to the performance level of my 7770? and if so then it would be a waste of money and I'd rather buy the same model.What do you think I should do?

 

[abizer14]

Best answer selected by abizer14.