I'm currently running 6870 Xfire. I can blow anything away in 1080p, but once I turn EyeFinity on, boy does she dog. I can get playable FPS for BF3 on medium settings, but that's about it. VRAM has to be the culprit. I've been thinking about selling them now and getting a 670 or 7950.
And, my reference design card runs 15C hotter than my non reference card.
In my opinion, it will be a significant boost to what you see now, but you may not be able to stretch their legs on the resolution you want. If you can't sell what you got and swing for the $400 ones, grabbing the second one is the next best thing.