Reports are putting the GTX560 at a little cheaper price than the 6950. With your resolution, 2gb is not necessary. Release date should be January 25th, less than two weeks away. AMD will release a cheaper 6950 1gb version as well. Why not wait, the 580 & 570 are great cards, the 560 is bound to be as well.
If you overclock to 6970 speeds (Clock/Memory/Shader) after unlocking the 6850 it will be faster than the 570 in most cases. I think the article said it was about a 20% OC... When you unlock the 6950 it doesn't increase the clock speeds (To my knowledge) so it will be an underclocked 6970.
Only thing I've heard is that unlocked 6950's don't overclock as good as regular 6970s.
There are also two different ways to go about it. One being more risky than the other.
You can flash to a 6970, and it's running at 6970 clocks unless you underclock. I've done this to good results and can even overclock it a bit. Because the memory is the more sensitive part of this flash process, I choose to Overclock the GPU and underclock the memory down to 6950 results. The end result is slightly faster than stock 6970 cards. Some people have had memory get damaged doing this. Although it looks to be a small percentage.
The other method is safer. The other method is to run a script that unlocks the disabled shaders only, and leaves the rest of the bios alone. This leaves you to use 3rd party programs to overclock, which I'd only recommend overclocking the GPU clock only. People are getting good OC's doing this as well. I believe they also are unlocking the max clock settings, so they can OC past 6970 clocks on the GPU clock only.