I've gone back and forth in using the more recent cards (I haven't used the 500 series yet), but after using the 470's and a 6950, I've been most happy with the 6950.
The things that I like:
+ The 6950 has better AA. In some games, there is a big difference.
+ The 6950's 2gb of ram has prooven to smooth out areas of games that used to stutter a little or drop in frames. (it could be another aspect of the card, as they do show to have better min FPS).
+ It's quiet.
The advantage of the 470's was that it has PhysX, which I only use in one game, and I don't particularly care for that games use of PhysX (Sacred 2).
I know I haven't tried the 570, but for the most part, it's just a bit faster than the 470. It doesn't suffer the same noise issue, but it still lacks the superior AA options and 2gb of ram.
I think the 6950 is a thumbs up, especially after I was able to flash the 6970 bios on it without a problem.