This video shows two similar 2-monitor setups: the first using PCIe graphics, the other using Intel chipset graphics + a DisplayLink USB chip like the kind used in Plugable’s products.
The DisplayLink solution takes the power consumption from ~80W down to ~50W. Over time, and as you add more monitors, that’s a huge savings.
Read more about this at http://www.displaylink.com/power_saving.html