I actually transitioned from an HD5970 to a GTX 570 and felt it was an upgrade. To be honest, the drivers are what drove me away. But make no mistake about it though, when the 5970 worked, it destroyed my GTX 570. So I just got a second 570 and haven't had a need to go backward since.
Ultimately, what a good upgrade is for you will depend on what your actual budget looks like. All I saw was the price ranges for the cards you are looking at, but not how much you are actually going to, or planning to, spend. If I missed it, I apologize.
Generally, for a single 1080p screen, an HD6870 or GTX 560 would more than suffice while not being as harsh on your wallet. Further performance can definitely be had if you add a second 460 for SLI as an even more cost effective solution. But if you're intent on spending money in the used 5970 realm, then a new HD6950 or GTX 570 would be the better choices over the 5970. If nothing else, at least you'll get a decent warranty as most 5970's are approaching the 2 year mark.