People had bad luck using those cards but they are as good as (better than?) others.

What kind of 7970 do you have ? I'm running a rig with 5 Sapphire 7970 VaporX ( 11197-12-40G ) and at first it was a slooooow 550 kHs / card... Bad.
Now I can get 700-710 and its stable as a rock.

The key was :
1 - flash the card bios ( for me Sapphire.HD7970.3072.120926.rom was the right one )
2 - Update system drivers to amd-catalyst-13.12 and amdapp_2-9 (I'm runnig Linux, don't know if it's the case for you).
3 - Those settings are important :
"intensity" : "13",
"thread-concurrency" : "8192",
"gpu-engine" : "1040",
"gpu-memclock" : "1500",
"gpu-powertune" : "0",
"gpu-threads" : "2",

Ask me if you need more informations about flashing the bios, its not an easy part if you have not done that before

Take a look at my rigs on my pool main page

https://www.mining-pool.co.uk/