1. Hard Drive: WD Caviar Green is the first step in putting together a greener computer is to simply go for greener hard drives. Hard drives spin to access data, so it make sense to try and find a greener option. Solid State Drives consume about half the power of a standard hard drive. Most SSDs use 1.9W of power in active mode and 0.6W in idle mode, which means that power and head loads are minimized. However, expect to pay for these types of drives, as they usually are more expensive than regular hard drives. The best solution would be to combine an SSD with a low-wattage hard drive, using the SSD to install Windows and all of you program files, and storing your data on the other hard drive. It would significantly improve your system's speed.
2. Memory: Kingston and Micron make low-voltage memory that will fit your motherboard. These types of DDR3 RAM will use about 1.5V, which is about 30% less than usual memory. In order to appear significant, these types of memory need to be combined with a low-voltage CPU, motherboard, and graphics card. Otherwise, the energy savings are simply lost.
3. Processor: Many computer rigs are just built to consume power, gaming rigs especially. It's not easy to find something that will consume less power and still perform well. The AMD Athlon II X4 processors consume about 45W, so they're made to consume a lot less power. The new Intel i7, i5 and i3 come in versions that consume less power as well. For the i7, the consumption is 45W, the other two models consume 35W.
4. Motherboard: The Asus M4A87TD motherboard is compatible with AMD Athlon II processors and also supports low-voltage RAM, making it a good low-power combo for a green PC. The Asus Sabretooth is a high-end desktop motherboard, but it features some interesting power-saving features, which could also make it quite frugal and efficient for a green setup.