Re: GTX570 or Ati 6950 ?
They are pretty equal cards in overall performance with a slight advantage to the 570. Why didn't you like the 5850? If it was because of software, then you're not going to like the 6950 either.
In my limited experience I haven't played a single game that was made in the last 4 years that crossfire didn't work in. I'm not saying there aren't games that won't work for me in the future, but so far It's been great. So take what you hear about xfire with a grain of salt IMO.
It would come down to price to me. If you don't mind paying the little bit extra for the 570 then there you go get it. Also keep in mind you can unlock the shaders in the 6950 that will give you a small boost in performance.