a 7870 would do u great, I went from a 6870 to a 7870 and couldn't be happier. A 7870 will be better at handling 1080P at higher settings than a 7850. Its def worth saving another few dollars to get a 7870
Depends on the game and how well the 6850 will scale. A very good case would be 90%. So if two 6850 are 190% stronger than one; it equates out to 60% stronger than a 7850.
I used both GPU's rated gigaflops of processing power for the calculation. Consider it a rough guide, as flops aren't "everything". And you may not get exactly 60% more fps playing your favorite game, vs. a 7850.
And just for reference the 6850's in crossfire work out 10% stronger than a 7870.