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I want to share with you my experience getting Sapphire HD 7970 over 700kh/s. I have 4 of these cards on my rig and i was struggling almost a two months to achieve 700kh/s. Yesterday i finally did it. First thing i want to say is that i tried many different configurations from many threads on this forum and elsewhere, but unfortunatelly, non of them worked fine for me (but i have learned many things from them, that help my solve my problem)
I was getting 520kh/s per card, with 1010/1375/1.025 (engine/memory/voltage) - when i was buying that cards i was expecting something about 720kh/s.
My settings in CGMiner was --thread-concurrency 8192 -I 13 -w 256. I read somewhere that thread-concurrency for 7970s should by around 24000 of less. But i was unable to get above 8192. When I tried to set higher value CGMiner reported some error message with buffer - i dont remmember that exactly. Almost the same when I set higher intensity. With overclocking i could get only ~560kh/s. The 0.5 - 0.65 engine/memory ratio did not work for me. Example from SCRYPT-README.txt did not work for me neither. -g 2 makes me hashrate drop down... Well, i was desperate
After some time I have found a post saying that I should have at least the same amount of system ram as i have gpu ram for scrypt mining and that system ram matters with scrypt. (It is actually written in CGMiners SCRYPT-README.txt - if I just read it from beginning to end, i should save so much trouble...) That time i had only 2gb of system ram, and each Sapphire HD 7970 has 3gb. I does not take long and I bought 8gb ddr3 ram. It makes a difference! Finally i was able to set higher thread-concurrency values. With that i could try again 7970 tuning example for SCRYPT-README.txt. The default thread-concurrency for me is now 22400 and worksize 256. My hash "skyrocket" to 540kh/s
Not what i have expected. But with this new ram i was able to set higher intensity. So with -I 20 i was getting around 600kh/s. That was better. With memory overclocked to 1500 i was getting 660kh/s. That was much better! But still - friend of mine was getting 720kh/s with the same card, with no problems. He just connected his fresh new card to his motherboard and it give him 700kh/s just like that 
Damn, Lucky bastard 
But i did not give up.I after reading some articles about 7970 overclocking i finally did what i was afraid to do. I change voltage. That brings whole new dimension to my overclocking. Now i was able to overclock memory above 1500 - to that numbers i saw so often in posts from other users. But it was no use. Even with memory to 1700 and engine to 1100 i was getting even lower hashrate that with defaults. I went back to 1500Mhz for memory. I tried to go up in little steps but when i went to 1501 - hashrate drops. I tried to go down a hash rate was relatively stable. It was almost stable within range 1450-1500Mhz - just few kh/s difference. I leave it on 1500 and i start to change engine clock. By the way my voltage was 1.135. I have started at Memory/2 Mhz and went up in 5Mhz steps. Hashrate was slowly growing but it was not significant. I passed 0.6 ratio and hashrate was still low. I start making bigger steps and when i was around 1000Mhz I was at 670kh/s. With 1030 i was at 693kh/s and finally with 1040 i was at 700kh/s. What a feeling!
I went up to 1100Mhz with ~730kh/s. I think if i change a voltage a bit higher and continue to going up with engine clock it could be even more, but i have stopped there because the power drain was to high. After that i was lowering voltage and trying to find lowest stable value. It was around 1.080V.I have tried all 4 cards at this settings but power output was ~1150W (I have 1200W 80Plus Gold PSU). That was to high for me. I lower the settings to 1060/1500/1.065 and power output was ~1075W and hashrate ~715kh/s. Again, I went down for a bit just to take load from my PSU and i ended at 1040/1470/1.050 with ~700kh/s and ~1020W. So the engine/memory ratio in my case is 0.707 or higher. I mine with this settings over 24 hours now and it is pretty stable. Temperature is little higher but that is expected with higer voltage.
I am so happy i did it over 700kh/s with these cards
and i hope my experience can help at least some of you, fighting the same fight i did. I hope it was not boring and I am sorry for my defective english. To complete my post here is my pc configuration and some picturesMotherboard: ASROCK 970 Extreme4
GPUs: AMD Radeon HD 7970 (Tahiti XT) (925MHz), 3GB DDR5 (5500MHz)
RAM: Crucial 8GB DDR3 1600MHz CL9 Ballistix Sport
CPU: AMD Phenom X2 545
PSU: Fortron Aurum Pro 1200W
Windows 7 64bit Professional
CGMiner 3.1.1
Drivers 12.8
APP SDK 2.7
Damn game 
What about buying some grenades? 
