I have run 3 variants of the 7950 cards:
Single Fan Sapphire 7950 -
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202030Dual Fan Sapphire 7950 -
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202006Tri Fan Gigabyte 7950 -
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125414I find that the Gigabyte is easiest for achieving 600 KH/s when overclocked to 1000/1450.
The other two Sapphire cards I had to push up the power % to 10% to get 600 KH/s.
That also may mean that the Gigabyte card draws more power. They do require a single 6 pin and a single 8 pin to power, vs the other two Sapphires that only require 2x 6-pin.
EDIT: And my settings on all cards are the same (I use afterburner MSI, works perfect):
Core: 1000
Mem: 1525
And on CGMiner:
--lookup-gap 2,2 --thread-concurrency 21712, 21712 -I 20,20 -w 256,256
If you want to ONLY use CGMiner to overclock, and not afterburner - use this:
--lookup-gap 2,2 --thread-concurrency 21712, 21712 -I 20,20 -w 256,256 --gpu-engine 1000,1000 --gpu-memclock 1525,1525 --temp-target 75 --auto-fan
(Note: the above settings on CGMiner are for two GPU's, adjust for 3)