1) Don't get a 6850. Get a 7770.
2) Get 8GB of RAM.
Actually overall in game and graphics wise the 6850 is better than the 7770, beating it in almost every scenario, although since the 7770 is the next gen gpu compared to the 6850 it is understandable that hardware based and such, the 7770 is the successor. As for pricing they can run just about the same but the 6850 is just a little bit cheaper. If you are going to want better in game performance go with the 6850 but as for (lets say) stress on your computer go with the 7770. I prefer the 6850 though but that is just my opinion. Also check this site out for any comparisons you need. http://www.anandtech.com/bench/Product/536?vs=539
Looking into this a little bit further, if you are able to spare a few more bucks, the gtx 560 surpasses the 6850 phenomenally for the price. One last thing, if you want to go the extra mile, get a gtx 560ti and try changing your build around to be able to squeeze it in. All in all I would prefer the normal 560 as there isn't much moving around you can do to your build. Hope I helped with the gpu bro.
I wouldn't recommend the 560Ti at its current price in the market. The 7850 is faster for the same price.
You do have a point there but they are both out of his price range which is why i recommended the normal 560. it is 170 right now and will get the job done on this budget build. You are right though, thanks for pointing that out.