Firstly, get an overclocking software like Riva Tuner, thats what I used. Certain video cards let you overclock them more while some let you less. I had Nvidia Geforce in the past, and they're more overclockable. If I remember right, it knows how to scan for the best values. If not, you can try overclocking more and more slowly until the image gets corrupt or the pc hangs up or restarts. Also, know that you may overclock it in windows and have it work fine, but in games it may get stressed more badly and corrupt the image there or cause problems.
If its the memory clock that's too overclocked, in games you will see surfaces getting offset the geometry and textures appearing at random and geometry shapes (like a car's headlight suddenly stretching to the sky). Unless that bothers you, its ok that way. The memory clock doesn't do much anyway so dun overclock it more then you need to. The core clock does indeed affect performance in fps drastically, but if you overclock it too much, you may get your video card too stressed and it may shut down or restart the pc. I played with these and never burned anything, but be warned that they can indeed burn a video card so do be careful with them. Thats mostly all about it I guess. There are other sub-settings of these too, but I didn't look into them much.