Tundra migh be right, as the 7870 is the maximum what the Pictairn can give, and the 7950 is an artifically downclocked version of the Tahiti.
Expecting all theese radeons to reach 1030-1050MHz on stock voltage (do we even need to push them that far?), the 7950 is about half way between the 7870 and the 7970.
Also, i apologize, havent checked the prices on Newegg, the 7950 is almost as expensive as the 7970, in Hungary.
I would still say, if you save some on the components like no water cooling, etc, it would be possible to build a 7970 CF. Maybe not from 1500, but 1600.
I wouldnt touch the Kepler right now, as the simmilarly performing Nvidia models tends to be a little bit more expensive then their Ati counterparts, also the Ati is way better in the OpenCl (computing performance).