Some games supports crossfire. Most modern games does. The problem wih cf/sli is microstuttering but latest drivers have improved this significantly.
I also own a 7850 2gb and am also wondering if it would be worth crossfiring but a single strong card will most of the time be the better choice because it will give you the best overall experience. If you're only playing modern games that support cf/sli and have the correct harware for it you may want to consider cf/sli.