I'd definitely sell the 6850 and get a single 7850. Once you over clock it, I'm sure the performance gap will be negligible, especially at your resolution. It's also a lot simpler running a single card, and you can be sure the power draw/heat/noise will be a fraction of two 6850s!
heat and noise is not an issue when a decent psu is in the front of any two cards, if you want to run those games on max setting then sure get another 6850.
You'll be see a massive increaase in performance, BTW what is you other specs?