Well if the only game you are playing now is WOW, then I wouldn't worry about CrossFire or even an expensive GPU. However, generally speaking, at least in my opinion, you get more of a performance increase per dollar increase by crossfiring two lesser cards than buying the one most expensive card. But it just depends on your budget too.
After crossfiring 2 4850 1GB's, the increase in price usually outweights the increase in performance. However, you can get the 4850 X2 2GB instead of crossfiring 2 4850 1GB's and get the same or maybe tad bit better performance for about the same price and still have a PCI-E slot left for another.
But again, with playing only WOW, you can probably get the lowest 4800 series card for real cheap to hold you over and then you can get a better and more expensive card later when the time comes that you need it and it'll probably be cheaper then.