The following are some useful resources on PC Power Supplies and troubleshooting them:

Power Supply Fundamentals
A Guide To Computer Power Supplies
Power Supply Fundamentals
Checking your PCs Power Supply Voltage Readings (good)
PSU Troubleshooting with a Multimeter
Computer Boot Problems
Upgrading to a High Efficiency Power Supply
Real System Power Requirements
Power Supply Connectors

A few tips on buying a PC power supply:

When Hardware [...]