If you're using Precision X, enable Kboost, move the power target slider all the way up, put +20mv on the voltage slider, put +100 on the core, and +300 on the memory. Adjust from there, depending on what you see in GPUz. Target clocks for a 970 on air should be around 1500 core clocks, and 2000 memory....each card is different, and you'll have to mess with voltages and power limits, but those are good targets to shoot for.
There's nothing at all in the stock bios that will allow you to harm your card in any way, shape or form. The voltage is locked in the bios to max out at 1.262v (on most 970s, some are less). You'll likely see a max of 1.256v. So, as long as your temps are good, move the slider all the way up and run with it.
Also...download GPUz and open the sensors tab. Use either Unigine Heaven or 3D Mark Firestrike to put stress on the GPU and test the overclock. That's not likely to give you "game stable" overclocks, as each game is a little bit different, and each driver version even more different, but they'll give a basic understanding of how to fine tune your clocks for the best performance, so you can do it for the games you want to play.
Now that you have GPUz downloaded, and the sensors tab open, watch the core clock, memory clock, voltage and pay particular attention to the perf cap performance line. The perf cap performance line will tell you exactly what's capping the performance of the GPU, so you can work around it. VREL, isn't anything to worry about. Really common. VOP, means you need more voltage to go any further. And PWR, means Power. You're at the cards maximum power target, and clocking any higher won't do you any good, because the card will just throttle itself to get itself back under the power limits.
There are ways around the power limits, but.....that's a whole different animal. Tackle this one first.
Oh...and IF you're running SLI. Open two instances of GPUz. One for each GPU. That will be important to dialing in your overclock to work well on both cards. The selector at the bottom of GPUz will allow you to select which card each window will monitor.