I choose factory-overclocked graphics cards for three reasons:
1. They have better cooling systems
2. They usually have a non-reference board design with better power systems (more phases = better overclock)
3. They usually have cherry-picked GPUs and memory chips that can handle higher speeds
I overclock beyond the factory-overclocked speeds. My GTX 570 is currently running at 860MHz core and 4200MHz memory. That's at 1.1v core voltage. I'm going to see if I can get it higher with 1.125v core voltage soon.