This definitely speaks to me.  I work in the internet hosting field with a small provider trying to green the way our business sector exists.  There are a TON of issues related to the sustainability of using this machine I am typing away at right now.  We are working on a real analysis of the ecological footprint of the computing business, but for now our research shows the following as the best ways to begin to reduce the impact:

1) Buy used.  As Ryan points out there are "old" computers out there that run perfectly well.  This is true for Mac, Windows, Linux, Unix.  RAM (Memory) can help a ton in making the performance better on an "old" machine.  Buying used and using things until they are truly not useful anymore extends their life... and thus reduces their relative impact over time.  Meaning you are absorbing more impact, more quickly if you use something for two years instead of 10.   Good sources are your local craigslist.org and ebay.com, always be a careful buyer of course.

2) Use an LCD for a monitor, instead of the chunky CRT.  You can easily buy these used.  The energy savings is huge...and did I say you can buy them used.  And they are also emit less electro-magnetic radiation and are easier on your eyes.

3) Sleep mode.  Powering your computer down when not in use is huge.   Mac is better at this then Windows.  Check out:

http://www.grist.org/news/maindish/2007/02/07/vista/index.html ;

...

As far as the issues about chemicals and heavy metals.  Computers are literally made up of hundreds of them:

"A UN research group estimates that manufacturing a 24kg PC (with a monitor) takes energy equivalent to 240kg of fossil fuels, needs 22kg of chemicals and 1.5 tonnes of water."

Beyond the one's getting most "greenwashing" by the computer manufacturing groups (Lead, PVC, polybromated flame retardants)... you'll find arsenic, mercury, cadmium, chromium, ...lots of petroleum by-products, etc.  So much so that there's even data that details increased mortality rates for people tht work in the computer manufacturing industry:

ref:

http://www.ehjournal.net/content/5/1/30/abstract/ ;

http://news.bbc.co.uk/2/hi/business/6110018.stm ;

So, needless to say, there's a lot we can do.  The simplest of which are use your computer wisely, use it to effect the change you wish to use, use it and reuse it... and buy new only when it is absolutely necessary.