My rule of thumb was always to spend at least 10% of the total system cost on the power supply. There has been a lot of improvement in power supply technology over the last ten years.

The most important element of the power supply is its ability to deliver stable voltages along its rails. You don't want the +3.3v rail spiking to 3.5 or down to 3.1, or the 12v rail dropping to 11.8; this is what eventually kills components.

Stability should be the most important metric a power supply is measured on. :) Seasonic and Corsiar equipment are very good in this area.