The best way to overclock your VGA is simply by doing so.
Download GPUz and use the monitoring tabs to keep an eye on your temps and voltages. But realistically you should not have to do too much if any voltage adjustment to get to the 7970 kind of performance. Memory overclocking is very good with these cards, but you need to strike a balance between core and memory.
If you leave your voltages on stock then just increase your frequency in steady increments until your system freezes when you do your VGA stress test. Run a benchmark like Vantage for example. Or Unigine heaven.
Then once it falls over, increase the voltage slightly. Now pay attention to the temps while you run the benchmark or game your favourite game. Increase voltages and frequency until you find your sweet spot or you get concerned about temps and noise. Remember you can increase the fan speed with Afterburner too.
That is how I do it.
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