When overclocking your gpu two things can affect performance. If temps get too warm or you hit the power limit, clock speeds will start dropping.
So before overclocking, it's a good idea to set a fan curve. At the top right click "User Mode", under Fan, click the little pencil and it will bring up the Fan Settings page.
As an example, I have the fans set to start ramping at 50c, and fans at 100% when temp hits 80c.
Click Save once configured.

To overclock,
Max out the Power Target, you can max out the GPU voltage too, Nvidia has the voltage capped so you'll only get another .03v at 100%.
With GPU Tweak III open fire up a game, ALT+Tab out of the game and raise the GPU BOOST CLOCK one notch and click apply, ALT+Enter to return to the game and test.
If all seems good, repeat the above step until it crashes or freezes, then back off the GPU BOOST CLOCK a notch or two.
Once you find the maximum GPU Boost Clock, do the same with the memory. Raise the Memory Clock in increments of 100MHz and test, don't forget to click apply each time you make a change.
On the left side is the Monitoring tab, we want to pay attention to mainly the Maximums of the GPU Boost Clock, GPU Memory Clock and GPU Temperature.
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