As rgd said, every card is different, therefor you're the only one who can figure out how far you can overclock your current GPU.
You can check varies GPU benchmarks for Windforce 970 and get an idea to how far other people could push their identical card.
Download Heaven Benchmark, open it in Extreme Preset (Window mode), and start by adding +20MHz on core clock, let it run for a minute, if no crash, add another +20MHz. Do this until you experience any artifacts or crashes. When you get any artifacts or crashes, dial back your core clock to the last known stable clock.
Then put your core clock to default, and do the same procedure with memory clock.
With memory clock, you can add more at a time, +50-100MHz.
Now put memory clock on default, and add your previous core clock overclock, and start by adding +50-100MHz memory clock until you crash. When you do, dial it back once again 50-100MHz and there you have your stable overclock.
Now test it in various games, other GPU benchmarks etc and make sure its 100% stable. If you experience any crashes anywhere else, dial it back a notch.
** Note: The above is just a rough idea how it can be done. Even better, watch some youtube tutorials about how its done if needed.