I currently have two HD6850 cards in Crossfire, but have started to grow a little bit tired of a dual GPU setup, with so many games having some issue or another. If it isn't flickering it's other annoying stuff. Many games do get fixed after a while with a new Crossfire profile, but some games just keep having issues.
My question now is this: If I choose to go back to a single GPU setup, what card should be needed to get overall performance at or above the level I get with my dual 6850's? I could go for either AMD or nVidia, but cost is an issue. Also, my native resolution is 1920x1080.
Today I get perfect performance in Bad Company 2, with it pretty much maxed out. Skyrim runs very well with a fairly high setting. Diablo 3 for some reason is having a few issues, like some micro stuttering, but in between that it runs smooth a butter.
The Secret World is brand new when I'm writing this and simply wont work with a dual GPU config. Something I hope will be fixed soon, but for now I'm forced to run that with Crossfire turned off. And performance then is so-so in DX11.
Putting into account the issues that often come with Dual GPU configurations..the 6850 especially, I would be comfortable in saying even something like a 7850 would give you very decent performance. The 6850 CF is more powerful, that's granted, but you won't have the headache of microstutters and scaling issues.
However, if you can spend a bit more money, I would agree with dalmvern in the suggestion of a GTX670...shortly followed by a 7970/7950.
You'll be hard pressed to find a single card that can produce the frame rates of two mid range cards like the 6850s in Xfire. But living without the micro stutter will be such a blessing, you won't mind. The dual GPU cards would be the only way to match or exceeed what you have now. I speak from experience. I had 2 HD 6850s in x8 x8 Xfire. Great frame rates but too much micro stutter. Go with the fastest card you can afford.
I've heard way too many problems about 6850 cf setups. So I can understand why your are having problems!
My suggestion would be to get a single card which is more stable. You can sell both of the ones you have, and be able to buy atleast a 7850. (If it was me though). I would save my money and then buy something like a 7950.