I love how we're giving answers to an OP who has not been on for 3 months
For those newbies interested in mining a 7950 can be undervolted and would typically run just over 200W, so you could run 4 comfortably with scrypt. The more energy efficient coins are even better.
One thing to consider though, every 10C of ambient air temp will pull another 10% of power form the card. If you're running them in summer the resistance from heat and increased fan usage causes more power draw. Hence winter rocks!