How long your video cards for the most part, relies almost entirely on the power being provided to it. You will have the best success with clean power from the outlet, from the power supply, from the motherboard and finally from the voltage regulators on the card itself. Many people think its the temperature itself but high temps just put your voltage regulators closer to their highest tolerances, which then results in failing components
Unfortunantly if you don't have a (high quality, line active) UPS, ensuring good power from the outlet is difficult or just impossible. I am very skeptical of cheap (<$20) surge suppression devices that a lot of people will think are actively doing something, when in fact they are just a $0.50 MOV or two and some wires on the inside.
The AC side of a quality switch mode PSU is usually pretty good at sorting some kinds of problems with the AC line. Many of the expensive ones can these days can even shut themselves down if they detect strange transient voltages, to protect themselves. There should be some good energy storage on both sides of the high frequency transformer, that helps give good smooth DC power on the output.
For example my Corsair HX1000 has even survived ~1 second complete power outages where most of my other computers and appliances have shut down, no doubt thanks to the huge electrolytic caps that must be inside it, and with not even 1% change on the DC output mind you!
Going on ebay in a week or 2 
