3870's will almost always beat a 4850. You should compare a single 4870 to a 3870x2 for closer performance.
Also, two 3870's can do 20k in 3dmark06... Think not? Check the orb. My rig hit 19,400 in 3dmark06 with my single 3870x2. Just for comparison, in the same rig my older 8800GTX would do...