Coding Horror has an article about cost trends in computing. Basically, it says that the cost of computer hardware tends towards zero (when you look at computing power per dollar), and because of this the cost of electricity is becoming higher and higher, relatively speaking. As a consequence, the goals of environmentalism and economic efficiency start aligning, which is great.
At the end is a very interesting quote related to gaming:
Most desktop PCs barely use 200 watts of power. It's extremely difficult to build a desktop computer that uses 250 watts of power without adding a high-powered $300+ gaming class video card to the mix -- or even two of them in SLI mode. Furthermore, you'll only reach that level of power usage under extreme load-- with the video card and CPU both operating at near 100% usage. In other words, only when you're playing a video game. The difference between idle and gaming load power usage can be more than 100w.(The quote in the article contains links.)
It's kind of obvious. Still, it's not something I was personally aware of every time I play a game.