Well regardless of it will work or not it would be a waste of money to buy a better gpu to crossfire with your 7850. I would suggest either buying another 7850 or a 280x to replace it. Here's the compatibility list.
Yes a 7850 will crossfire with a 7870 but the 7870 will drop back to the 7850. I am running this set up. When I ordered my second card, the 7850 came. I do not know if I made the mistake when I ordered or if the 7850 card was a substitute. If you are planning this I would go with the 7850 to match the 7850 you already have. To be compatible in crossfire both cards have to be in either the 7800 series (7850, 7870) or both in the 7900 series. The 7800 series is not crossfire compatible with the 7900 series. The compatibility list that wander11 has shown a link to will show this.