There is no point in buying 7970 for mining. It offers no benefits over 7950, while 7950 is cheaper, delivers same MH/s and drain less power which is impossible to ignore when talking about efficiency ratio
BTC/Watt [when you pay for electricity].
One thing which I want to point out. If you can, please avoid cards with 80mm fans (if you work/sleep near PC anyway
). Those are realllly noisy when card is running at full tilt. I know, bought 2 7950 MSI TF3. Jesus what a racket. My GTX580 TF3 (was using it previously for little mining) is nowhere to be heard, but 7950s mounted in same case just klanking like passing nearby Heavy Panzer Battalion.
