No high end GPU's are not always best. If you have a poor CPU then you will get what is called bottle-necking from your GPU as your CPU tries to keep up so to speak.
Most GPU's these days can be over-clocked but to be honest I have never seen the point as you really will not notice the difference in most games.
Not sure what you mean by customisation when you refer to graphics card.
On my last build I decided that I would try and build goodish gaming PC but to do this it is not just the graphics card (GTX 670) but CPU (I5 2500K) 8GB RAM and a Decent motherboard. So it really is several components being compatible with each other.Not just a case of bunging one decent component in an otherwise mediocre PC.