It was widely known for a long time
that an overclocked GTX560 Ti is as fast as a GTX570. From many reviews we also know that a GTX570 is slightly faster (2-3%

than an HD6970 at 1920x1080 4AA. Therefore, if you don't need 2GB of VRAM for 2560x1600, the GTX560 Ti overclocked = HD6970, not 6950 2GB.
On the NV side, you can pick up a solid overlcocked GTX560 Ti with lifetime warranty for $
230. On the AMD side, you can pick up an HD6950 2GB with Dirt 3 for $
240. Both are great options depending on what resolution you are gaming at and how much power consumption matters to you. Neither card is a clear winner.
But you can also pick up a stock GTX560 Ti with lifetime warranty for just $
195. Overclock it to 950-1000mhz and you get HD6970 level of performance. That's cheaper than most 6950s.
At the end of the day, the performance is so close, just pick based on the games you play. Crysis 2? NV. FEAR 3? AMD.