This is something that I've been thinking about for a while,
There are large pushes, all over the globe, to go "green" or to use more energy efficient devices.
What does this mean for a PC user?
Perhaps it means using a slower PC, one that requires less energy to run.
But, as a gamer, speed is indefinitely an issue.
Perhaps it's more down to using the right tool for the right job? a PC to run mid-range to older titles and a PC to run the heavier hitting games available?
Somehow, I don't believe this to be true, because even if you're using one PC that is more "green" you'll still use your regular PC on top of it, meaning you'll be using more energy than less.
In my opinion, the gaming industry cannot afford to produce slower models for benefit, but they can attempt to achieve the same speeds while using less energy. That is how I feel the market should go.

And it will go that way. With our fossil fuels depleting, we'll turn to other sources of power like hydroelectricity, and companies will have to create electronic devices that can maintain the same amount of speed without using all the energy. I don't see using computers less; we're actually going the opposite way, relying on them now more than ever. It will continue to develop that way, but engineers will find a way to nurture such changes. The age of technology is not ending; in fact, I'd say it's still beginning.

Considering that the industry is having trouble keeping up with the rate of speed increase, that is already the direction they're going in.

Add to that the energy required to manufacture 2 PCs rather than 1, and you probably won't be doing the environment a favor anymore.
That's how it's already going, and has always been. The industry is constantly working on lowering the power requirements, and for a very simple reason. More power used = more heat generated. Just in order to keep laptops as a viable alternative to desktop computers, they need to be able to make chips with a significantly lower power consumption than in a standard modern desktop computer. There are other reasons as well.

LCD making with Nitrogen trifluoride is worse for environment than coal or using power hungry PC..
Nitrogen trifluoride's globe-warming effect reportedly could be 17,000 times stronger than that of carbon dioxide
The growing market for flat-screen televisions is the real threat for nature

I have two computers. Well, three. I have a laptop that can play FEAR in medium quality, a PC that kicks some serious butt (8800 GTX, 4GB RAM, 2 500GB drives in RAID, etc.), and a white Intel iMac. I use my PC for work/gaming/anything at home, but I have to lug my laptop to high school so I have work and games for "improvement". My iMac is just there, and I use it as a footrest (though I do use it for some Photoshopping and LAN parties. My PC obviously uses the most power, but that's just me.

That IS the golden grail and is always being strived for. With every core change the voltages dsometimes do go down but some Gfx processor changes increase the power consumptio changing the ratio again.
You could do your bit, if you're feeling guilty, is offset that by switching off unused appliances.