Well, if you get the newest, you're probably set for the next 2-4 years. The other, maybe 1 - 2 years. Then you're not going to be getting the highest settings unless you're being crafty with your setup.
Plus think of it this way. If you were going to get Borderlands 2 (I hope you were, it's going to be great), then that would be $50 at launch. So really the 660 is only $240. Or does the 560 come with Skyrim?
for good deals.