What are you looking to spend? If you can afford the 7850 then you should get it , if you can afford the 7870 then you should get that. The thing to remember is that with gaming the single most important part is the video card and you should never shoot for what is just good enough because then you'll be upgrading the card every time a new game comes out or your situation changes and you get a higher resolution monitor.
here is no over kill with a single card, you only get that with two and three cards and the dual gpu cards. With a more powerfull card you are able to use more and higher settings like adding 8x faa instead of 4x and so on.
So it all depends on your budget and what your willing to spend.