The HD 6950, priced around 300$ performs like the HD 5870, while the HD 6970 priced at 370$ trades lows with the GTX 570( the MSI one is priced at 330$ on Newegg) at resolutions upto 1920x1200, above which the HD 6970 pulls ahead due to its 2 GB framebuffer.
However the 6950 can be unlocked to a HD 6970 through a BIOS flash.This seems a nice option and has indeed worked for many, but you can't say that it'll unlock all the time.
The HD 6xxx series has got improved tessellation performance, but the GTX 570 is still better in tessellation intensive games.
If you want to spend the entire 300$, get the HD 6950, otherwise if you want something for less, I suggest that you wait for the GTX 560, which should be able to match the HD 6950 in terms of performance and is arriving on January 25th.