Yes you have to run at least 2 cards, which is recommended anyways for performance with high res multi displays. You might even have to run 3 cards because I think some cards only support 2.
Edit: Actually i lied there is a special card they made for this http://www.newegg.com/Product/Product.aspx?Item=N82E16814129262&nm_mc=KNC-GoogleAdwords&cm_mmc=KNC-GoogleAdwords-_-pla-_-NA-_-NA
I don't think its worth it because at this price, you could almost get two of the regualr ones and crossfire them for more performance
So my recommendation is get 2 radeon 7870's and xfire them. But if you have a big budget (which to me it seems you do), you can get 2 7950's or 3 7870's OR 3 7950's!!!