The 7990 will not affect the price of the 7970, Nvidias 680 & later 780 should, but how much will depend on how Nvidia prices them and noone can do much more than totally guess at the moment how much. I suspect alot but could be totally wrong. If this from the sticky at the top of the forum is even close to accurate (and performance is anyware near whats expected)then you will see a massive drop.
Just because the 7990 comes out doesn't mean theat the 7970's peice will drop , you might see a price drop or rebate/sale option when the Nvidia 6xx is released. At the moment AMD has no reason to be making price drops since Nvidia has not made any info available for thier release date. I still see the 6990 for over $700 and almost all of the GTX 580's are $500 or more.
As it stands right now until nvidia releases the keplar line ATI (AMD) has no need to drop the price of the 7970, where i work customers are willing to pay that price because they are the strongest single GPU card on the market (gaming wise). ATI might as well for the time being milk people for their money because they are in the position to do so.
The HD 7990 will be priced higher than the HD 7970 so there should be no decrease in price.
If you want the HD 7970 for a lower price then just wait several months for the price to slowly decrease. Depending on the performance level of the nVidia GTX cards, the price may or may not come down.
The 6990 and 6970 are still pretty darn close to the price when they were first released. The prices don't tend to change. The cards just get phased out. Even if you go to ebay used cards are still selling for close to the new price. It is kinda nuts.