More than any other single computer component, the monitor is a power drain. In a typical system, the monitor accounts for at least half the entire energy draw [source: ACEEE]. So if you have the means to upgrade to a more efficient monitor, that's the way to go for real energy savings.
The difference between a high-efficiency and a low-efficiency model can be pretty drastic. Even the difference between two efficient, Energy Star-qualified monitors can be pretty mind-blowing. For example, the Philips 150S7, a 15-inch flat panel, uses 12.8 watts in Active mode. The ViewSonic VG510s, also a 15-inch flat, consumes almost twice that: 22 watts. And both are Energy Star monitors.
Both consume about 0.8 watts in sleep mode, though, which is low, even by Energy Star standards. Sleep energy use is often where you'll see the biggest difference between high- and low-efficiency monitors. And since (if you activate your power-saving modes) your computer will be spending lots of time asleep, upgrading to an Energy Star monitor with a super-low sleep wattage makes a lot of energy sense.
For more information on energy-efficient computing and related topics, look over the links on the next page.