6870 crossfire will give you more fps with some drawbacks
2 6870 on crossfire will use much more power than the 7850(also 7870 uses new architecture than consumes less power).
2 pcie slots will be used up instead of one
It will be hard to find the identical card you have now
If you have 2 pcie x16 slots and enough 6pin connector to power the 2 hd 6870 then you are good to go.