This San Francisco Chronicle article is one of the better compilations I've seen on taking computers greener, so a highly recommend read. Suggested steps include using power management on your PC (believe it or not, this step alone can save 90% of your computer's energy usage!) and going for the all-in-one solutions for printers and other peripherals. Other articles have had good points about avoiding the all-in-ones due to the difficulty finding one machine that can do everything as well and efficiently as the single-use counterparts, but when you think about it, replacing 3 or 4 pieces of quipment with one energy-star rated piece makes a whole lot of sense from an energy standpoint...
Disclosure: The author, Amy Sage, is employed by MyInkPro.com.
Awesome article green one!