Just buy that sweet 670 you will probably feel the need to upgrade in a few years still. But just because better cards come out doesnt mean you need it. Worst case scenario if somehow some new system busting game comes out by the time you need more gpu you could probably get a second 670 cheap.The 600 series is brand new so it will last a reasonable amount of time if your thinking of waiting you will be waiting a VERY long time.
Your logic does not make any sense at all, what happens when you buy the 700 series ? there will be another upgrades after you buy yours....upgrades won't stop, the 600 series is powerful and i believe its enough and will last a proper time,,,,