Over time, the power requirements for computers are constantly changing. In particular, the trend is upwards, with up-to-date graphics cards (or multiple graphics card systems) drawing very high loads.
A stable power supply which can deliver high enough wattage is an important part of a medium or high end system. The site below has an interactive power supply calculator, which is frequently updated. Note the comment on that site about +12v line amperage. This is particularly important for dual graphics card and/or overclocked systems.
http://extreme.outervision.com/
This page of the wiki has a list of recommended power supplies commonly available in New Zealand. This is important, because a number of psu manufacturers take liberties in the methodologies they use to rate their power supplies, and one manufacturers 500W is not the same as another. When in doubt, choose a power supply with a respected brand.
Credit for link in original GP Forums thread to SKyN!