apparently having imperfect/fluctuating power to the PSU slowly "erodes"the internal components. for example, when a powersucking appliance in my house turns on, the lights get dim for a second and the Mad Dog MultiMedia 350W PSU in my p4 system makes a funny clicking sound. however, its a relatively not used much psu so its not dying now(even with the c*ck-*ss fan mod) but it may be a problem in the long run!
A circuit that is getting up to its rated load will cause more resistance and with it heat in the wiring. If you have a typical breaker panel, most circuits are on 15-20 amp breakers. A dryer is usually on a 30 amp breaker, a electric stove a 50 amp. These are usually on a circuit of their own. A surge spike on a circuit will not always pop a breaker but can be very hard on sensitive electronics. The idea is to have a breaker kept under its rated load and sensitive electronics kept on a surge/ power conditioner. A whole house surge protection unit is very common now on new home builds. Utility power coming into your home can be very dirty thats why they sell 130 volt lightbulbs to help with small power spikes as opposed to just 120 volt ones.