Just curious on this topic since I've never owned an Sli or Xfire config. I plan to buy a second 7970 eventually, however I'm concerned about compatability. Would I get better / more reliable performance with two card running reference PCB's? Or does it matter at all?
I'm assuming varying clock speeds doesn't matter since I can match them in CCC, and fan design of course shouldn't be a problem.
the problem is really when you overclock the reference one...
to sync them you should downclock the non reference to match the reference that should prevent any problems from happening, it should not bring you problems doing reference + non reference, but yes it is something like 100% sure it will work with exact same cards, and 98%that will work with different cards.
so if you have a reference using another reference would be good because they have same clocks same memory size, same cooling etc... but it WILL work even with a non reference card but it is always nice to buy the same card.