The first lesson in power management on modern electronics is understanding that nothing is really off until you pull the plug. In my own personal computer lab I acquire a nice tan with the computers and room lights turned off. Routers, printers, surge protectors, backup power, speakers, and monitors all emit a radiant, friendly glow from their many LEDs. I have a running peeve with Hewlett-Packard over power switches on printers. My old HP6L doesn't even have a power switch. The newer Laserjet 3050 has a switch located on the back, way down at the bottom, requiring a long double-jointed arm to reach it. I'm waiting for HP to claim green creds so I can scoff at them.
The major power users on a PC are the display, the CPU or graphics card, hard drive, LAN interface, and power supply loss. Hardware vendors are paying serious attention to cutting power consumption, so you can factor this in when you're shopping. The Kill A Watt electric consumption monitor is a wonderful, cheap-and-easy device for measuring how power-hungry your gadgets are. It measures consumption by kilowatt hour just like the electric company, so you can easily calculate what your devices cost to run. It's nearly always an eye-opener to see how much juice your gadgets are really drinking, even when they are turned "off". It also captures voltage fluctuations so you can see how clean (or dirty) your power is.
Computers, like all electronic devices, last longer and perform better with clean power that is not beset with surges and sags. If Kill A Watt tells you that your power is flaky and unreliable you should consider using some line-conditioning devices, such as higher-end uninterruptible power supplies.