If I remember correctly, crossfire doesn't scale correctly in most games, especially if you don't have a X38 or X48 motherboard. As the 4850 is better than the 3870x2 in most games, it's probably better than two 3870s in crossfire.
You can get a 4850 for 150$ if you find the right website, or 175$ at most retailers...so just sell you old 3870 and get a 4850 with the extra 60$.
Edit: Didn't see you had a X38 mobo. You should find some benchies on how two 3870s in xfire perform against a 3870x2...and then compare the performance of the 3870x2 vs. the 4850. A simple rule of 3 should give you your answer