An upgrade is worth it when the card you want to upgrade to is going to make a good enough performance increase. A good time to upgrade ,with that in mind , is usually when two to three versions have been released. I f you wait for the 6990 prices to fall then you might as well get the new release (7000).
As far as I know all 6950's can be flashed to a 6970.
Selling your card for a certian price depends on how much you paid for it and how long you want to take to sell it , the more you want the longer it would take , and it is usually a personal thing of how much you feel it's worth and what you will ultimatly settle for.