Not a good idea. HD 7950 is selling for 300 with 3 free games - Farcry 3, Hitman absolution, Sleeping dogs. HD 7950 is 3 - 5% slower than HD 7970 at same clocks. HD 7950 has much better tesselation and compute performance. With a single GPU you can avoid micro stuttering. and other multi GPU issues like poor CF scaling in few games. Frostbite 2 engine performance (BF3, MOH Warfighter ) is poor on HD 6900 cards.
1920 x 1200
HD 7970 Ghz = HD 7950(1.1 Ghz) - 64
HD 6970 - 31
HD 7950(1.1 Ghz) matches HD 7970 Ghz (1.05 Ghz) and is slightly faster than HD 6970 CF (remember HD 6990 < HD 6970 CF) in most demanding games like BF3, Crysis 2, Batman AC, MOH Warfighter. Get the Sapphire HD 7950 Boost or Gigabyte HD 7950 OC. The HD 7950 even when overclocked will also draw less power and run cooler