This is what your MSI Afterburner should look like.
The first slider, the grey one, is the core voltage. If you increase the clock rate (the one below it) enough, you might need to adjust this to compensate. Just like if you tune your car for more horsepower, you generally tune your car to use more gas as well.
The second slider is what would directly affect the performance of your games. The core clock increases the speed at which your GPU runs at. Since the GPU is essentially a processor, this is like overclocking the processor.
The third slider is the shader clock and is usually linked with the core clock. Leave it that way.
The fourth slider is the memory clock. This allows the memory on board the GPU to run at a higher speed. It is good practice to increase the memory clock with the core clock as well.
The fifth one is the fan. You probably shouldnt be overclocking if you dont know what this slider does
To overclock, increase the core clock in increments and see if the computer is stable while running a stress test or a game. When it stops being stable (crashes BSOD etc) then clock it back to what it was before and leave it there. You might try to adjust the memory clock as well to get it stable.
Under the settings for MSI Afterburner, I would recommend checking "launch at startup". I personally would manually apply the overvclock, but you can set it to automatically apply the overclock as well in this menu if you get a very stable overclock.
All in all, I would still recommend a new card. Even a used card if your friends have any. A mid-range GPU from 8 years ago (8800GT) can out-perform that.