The BBC is running an article on the previously mentioned study on the environmental impact of computers.
A big part of this is that users are looking at the power factor. Obviously, a 4GHz processor is more desireable than a 1GHz, or *gasp* a 950MHz like I'm using. Guess what though- my 950 runs almost any app _just_ fine, and for gaming, my sweet ass video card makes up the difference- and really, my card is overkill for what I use it for. Anyway, back to the point, as you upgrade processors, the heat output of the processor increases rapidly. Even my 950 wastes a tremendous amount of energy in the form of heat.
And that's not even looking at the production factors. In making chips a tremendous amount of lead and mercury gets used. Behind the chip factory where my father works is a dead-zone dumping ground for these chemicals. He goes through haz-mat training on a regular basis to deal with accidential spills.
We're nearing the upper-end of what's possible with semiconductor technology. So it's time to refine semiconductor technology, instead of pushing for a faster chip. We don't need them- what we use our computers for doesn't need that kind of power. I rarely ever use 100% of my processor for more than a few seconds. I'd be more interested in cutting the power consumption of the computer I use.
Also, disposal is an issue. Really, that old 333MHz computer is far from useless. I've still got a 333 board in my closet- deliriumcrow is using a 333 for her computer- and yeah, she could use an upgrade, but there's still use for that 333. Instead of throwing it away, someone like me can come up with a use- _or_ it could be donated to a school, something in the spirit of the Boys and Girls club, etc. It isn't great, but it can do the important basic tasks if you're willing to settle for older software.
I'm kinda rambling at this point, but I think people can see where i'm going.