Just use MSI Afterburner, set the Power Limit to +20%, Core Clock to 1050, Memory Clock to 1500. Apply and enable the "Apply overclocking at system startup" at the bottom of the UI. Go into general settings and enable "Start with Windows" and "Start minimized"
Now your 7970 should always be running at the overclocked speeds
You will only need to increase core voltage if you start to get graphic artifacting, glitching, black screens or any anomalies. That means your overclock settings are not stable and the GPU might need higher voltage. (In general, avoid touching core voltage as it could permanently damage your GPU or reduce its lifespan)
I am using a Sapphire 7950 VaporX Boost/OC, passively overclocked to 1050MHz core clock and 1450 memory clock using the same method as above. Runs stable and doesnt exceed 60 degrees C. Extremely happy with it