Overall it's got to be said, at stock clocks the value of the HD 6870 isn't as strong as the HD 6850. With that said, it's also got to be mentioned that the only cards that offer better value when it comes to out of the box performance are the HD 6850 and GTS 450, with the latter really being in a different price bracket.
No doubt you're going to ask yourself the question, which model should you buy? - Like the GTX 460 and GTX 465, with the latter being one of the shortest lived models ever, the HD 6850 is going to be the better value product. But for out of the box performance, if you're happy to spend more, the HD 6870 is no doubt going to be the better option.
While AMD say that these models don't really replace anything, it's probably safe to say that after looking at both cards the HD 6850 would replace the failed model that was the HD 5830, and the HD 6870 replaces the HD 5850. For the most part the HD 5870 is still a faster card; you wouldn't move from a HD 5870 to HD 6870 as it would just be a downgrade; not much of a downgrade, but a downgrade none the less. This brings us back around to the fact that the HD 5800s aren't being replaced by the HD 6800s, even though it seems that two models could easily be replaced here in the HD 5830 and HD 5850.
What this ultimately does, though, is get us all excited about what the HD 6900s will bring; a series that is designed to replace the HD 5800s. If we're seeing this performance already, hopefully we'll see something extra special come November when the new top of the line single GPU models begin to poke their head around the corner.
AMD said they wanted to offer us more performance with less power; that's fine, but more performance and less power than what? - If they wanted to get more performance than the GTX 460 with less power, they've achieved that. If they wanted to do the same, but with the HD 5850, they've achieved that as well. I don't think that there's any way that AMD have really failed to accomplish their goals here, it's just that this last minute name change and lack of a real standing on where the models go in the market does make it a little more difficult to dissect then previous releases.
In the end we let the price do the talking and it seems the GTX 460 768MB is getting wiped out by the HD 6850 and the 1GB version is in trouble because of the aggressive pricing AMD have opted for on the HD 6870. An overclocked GTX 460 1GB with the help of a voltage increase doesn't warrant the price you'll pay for the model anymore it seems.
While there's really no way you would move from a GTX 460 1GB to a HD 6870, we have to wonder if you would bother buying a GTX 460 1GB now over a HD 6870. AMD hope the answer to that question is no.