ARS Technica published an interesting PC building guide in which they focus on power consumption:
For the home user, the benefits are more direct: a lower electric bill, both from reduced consumption and, for those in hot climates, less money spent on cooling your home office. The primary trade-off for individuals is the direct cost of upgrading or replacing an existing system to a less power-hungry one: you may never recoup the upgrade cost, even if the computer is run continuously. At $0.14/kWh, going from a computer that consumes 100W to one that consumes 60W would save $50 a year—which is many times less than what it would cost to upgrade the computer.
If your existing system is a power-hungry Netburst-era Pentium 4, the savings may be more dramatic than the example above, but even if you save $100 a year, upgrading a perfectly adequate computer for the sake of lower power probably doesn't make financial sense. At $300 for a minimal box, that's still three years to pay it off, which is a long time for a computer.
Hence, the real reason to go green is because you want to. Be it bragging rights over the neighbors, maybe your existing system is ready to be retired, or because removing 20 or 30 or 50 or 100 watts of power consumption in your study means you don't have to run the air conditioner, we're happy to see what's available for when you want to go green.
You can read the guide over here. First they build a pretty green but powerful gaming PC for about $1,078. This system uses about 83.8W in idle and 237.2W in load and features a Radeon HD 3850, Intel Core 2 Duo E8400 and a 20" LCD display.
Next they build an extreme green box with a VIA motherboard/CPU, this system uses between 58.4W and 97.3W but costs $1,438.81. However, the main reason for this high price is the $725 SSD, price/performance wise it would be a pretty good idea to go for a regular HDD instead.