The fact that it's underclocked doesn't really matter--it's easy enough to fix that using Sapphire Trixx or MSI Afterburner or any of those utilities (or even Catalyst itself). And it's still way faster than a 6670, even if you never touch the clocks. But yes, I did notice that (it's the reason for several of the negative reviews). The other 6670 for $90 is good too, but it's $90 post-rebate, so that might be a bit high. Let's cross our fingers on that shellshocker.
Yeah, and just to repeat, even though the 6770 might be close to double the price of the 6670 after rebate, it could be more than twice as fast, especially in new games. Look at the Diablo 3 chart with higher graphics settings: http://www.tomshardware.com/reviews/diablo-iii-performance-benchmark,3195-5.html