IT managers wanting to make a difference face two problems. Firstly, corporate bean counters won't wear purchases made on the basis of a fondness for hugging trees. This is why the WEEE directive is welcome, because it connects green issues to concrete cost factors. But the bigger problem is that environmental impacts are very hard to quantify.
The first I have covered repeatedly; profit and environmentalism have merged (See Yogurt Bombs.) The second is quantification of the benefits. Bingley makes a great analogy to cars by mentioning that the most enviromentally friendly car you can buy in America is the Jeep Wrangler because it is locally made and produced. Obviously the mantra Reduce, Reuse, Recycle applies here. You might want to read these tips on taking the lead on green IT as well.