At 1080P resolution, the 6950, GTX560ti, 6970, and GTX570 will do the job well. They are in the same performance category. There are differences between Ati, and Nvidia, depending on the game.
You pretty much get what you pay for because the market is very competitive. Any one will be appropriate for your cpu. Don't sweat the differences. Get the best card you feel comfortable, paying for; even up to the level of a GTX580. Only if you are planning on a 2560 x 1600 monitor or triple monitor surround gaming should you consider sli/cf.
As you get better cards, you of course get better frame rates, but the big thing is that the minimum frame rates get better, giving you more consistent gameplay.
I am not much in favor of overclocking graphics cards or of trying to unlock some of the capabilities of the 6950.