I just bought a 7970 widely available and in stock for £380... I would say if you looking to buy a GPU right now and want great performance then go ahead and buy it....
The way I see it is it will be the same... the 670 will be on par with the 7970 (maybe the 7970 slightly ahead) and the 680 will be better than the 7970 but cost more (around £400 I reckon) With the 570 and 7970 at about £350.
Throughout the year the prices will probably drop but I wouldnt expect them to drop instantly as soon as keplar comes out.... this is Nvidia were talking about, they are watching AMD make a killing of each card and they arent going to come in and undercut them completely they are going to stay with the high price/performance AMD have set and make some money....
Say you buy a 7970 for £380 now, its going to take atleast half a year before it will drop by £50..... and whats £50? less than a days work and for half a year of great performance thats great.
When Keplar comes the prices just arent going to crash that much.. they will obviously settle but the difference in terms of price/performance will only be like £20 less at the most I reckon compared to the 7970 at £380, Nvidia will want to make the most money they can, then throughout the year it will steadily drop.