The older a PSU gets, the less it is able to deliver to your computer. A lot of people have a two year old 400-500 watt no-name PSU that came with their computer / case. The problem with the PSU comes in when those people upgrade their video card to one of the new-gen cards (the 8800GT is a power hog) and their PSU is barely able to keep up. As the PSU continues to decline over time the higher the risk of the PSU or the motherboard getting fried.

This is why when you build a computer you want to get a PSU from a good company that delivers more power than you need at the time, so that in 2 years you won't risk blowing up your computer by putting in a new video card.