I am surprised that no one had anything to say about the upgrade, but you can close this thread, since I finally found a site that can directly compare benchmarks between both cards, thus giving me the direct answer if 660, 670 or 680 make sense to upgrade to from 460.
Even though probably no one will read this, I was particularly surprised by the absurd price/performance differential for 670. That 2 GB card costs 50% more than it should - it would be barely worth $240, definitely not $380 they ask for it...
It is crazy, that even after 15 yrs I have been buying gfx cards, the market is still so ****ed up and is heavily geared towards screwing the customer as good as it gets.
You can so easily buy a crap that costs 100-200% more than comparative alternatives, unless you spend a hell-of-a lot of time doing research across 10 sites.
It almost looks like the marketing department gets more guys than R&D department