6850 if not planning on cpu upgrade, 6870 if you are
Get 6870. Thats what I have, it runs BF3 on ultra with i5 2500k and 8gb ram.
6850
I have a 6670 1GB GDDR5 and it can run games on either high or best graphics at 1920x1080 with what I have been playing. Not tried BF3 or Crysis with it so no idea on those settings. Still I like the card so I recommend it if you want something cheap-ish.