I think a number of things come into play with OC'ing. PSU is actually quite important, ie being able to supply the wattage with enough overhead. The case was also a consideration, ie having enough air space to get rid of excess heat. And placement of machine to allow that air to circulate. There's nothing really special about my system, I just think it was well designed via my needs and the technicians advice. This was also my first OC setup.
I've always had custom built machines with good components, whether that being me building it, or in this case, someone who knows about OC'ing advising me. It may have cost an extra £200 for getting better components, but those components have worked out very cost effective. I tend to change my system every 3 years, and my mother still uses a machine I made 5 years ago, a 2.66ghz, my home machine was my last work machine, quad core 2.66ghz. Funnily enough, the upgrade has usually cost around the same price over the years, ie £900.