As far as i understand the 6850 is much better at tessallation.
So given the opportunity you'd be better off upgrading to the 6xxx series.
The HD6850 is a slower card than the HD5850. Tessellation and crossfire scaling are improved in the new series but even for both of those it is basically just enough for the HD6850 to catch up with the HD5850. Here is some benchmarks for the tessellation heavy Unigine Heaven benchmark:
The HD5850 still manages to squeak out a small win over the HD6850.