I would never count on "unlocking" a Graphics Card or CPU. Buy the card you actually want, it will save headaches in the future. Also, if the price of the 570 is too much to swallow, I'd recommend going with the 560 as PsychoSaysDie did.
Whatever you ultimately decide i would hold off till tues ( 560 release) regardless if you are not considering it or not the graphics card market and prices are affected at every major release so i recommend waiting then take a fresh look after tues or so i currently have the same decision to make and by waiting a couple of days you will have the best chance to make the most informed decision
I just got the 570gtx and I couldn't be happier. I replaced my 260 gtx sli setup and I am using less power, generating less heat and maxing out every game at 1920x1080 (except Crysis). From what I've seen in numerous benchmarks the 570 is a faster card in general than the 6950 but is more expensive. Two 560's may end up being the same or close to the same price as a 570 but I was looking for lower power and heat with a noticeable graphics bump.
I went from 470's in SLI to a single 6950 (flashed to 6970) and enjoy gaming a lot more now. I know I lost a few FPS in most games, but it doesn't matter most the time, as I get enough. Mainly I like SSAA and MLAA which the 470's didn't have (the SSAA can be unlocked with Nvidia Inspector, but it didn't function as well). The main reason I changed was I absolutely hated the noise levels of the 470.
A word of warning, about 5-10% of those who unlocked the 6950 had damage or didn't work. There is a shader unlock script that has no issues with people. You might find that safer.
As many people have already stated.. the GTX 560 is coming out this Tuesday. This would be a great graphics solution yet the prices will be pretty steep as release, and you get much greater performance from a 570.
do what I did. Find a local store that will match online prices and try to unlock the 6950 it you unlock it run a few benchmarks and play some game it will be very noticeable if there are issues you we see major artifacts. I went to frys purchased a Diamond 6950 it unlocked fine however there where artifacts. So now ill return the card and purchase the XFX model (because I found It cheaper not only to try to unlock again) and hopefully it unlocks. Wile my card was unlocked I played Crysis for a bit (before it started messing up) It was a noticeable increase and for free thats hard to beat.