You can't do eyefinity on two monitors, imagine if your playing whatever first person shooter, where would you crosshairs be? So, you need to have an odd number of monitors to do that. If your just going to extend the display so you play on a main monitor and watch Youtube or do whatever on the second one, that takes pretty much no power at all (I do that on my 7770) all it really does is make your card stay in 3d mode, meaning, higher idle temps. 7870 is a great card, for a great price!
its good for two monitors if,
you are gaming on one and let say watching netflix on the other.
not good for playing a game across two monitors acting as one.
but that card is able to push three or more monitors that act as one very well (some games) it just has to be more than two.