This is something interesting, a company has created an Atom based system board that can operate at just under 4.5 Watts.
The company's name is InHand Electronics, they put together a system board they are calling firefly. This is the world's smallest and most power efficient Atom single board computer.
To get this size they did not remove functionality. I have you have both onboard DDR2 and an expansion slot for an extra SODIMM.
In most current systems you need to have an additional power board to run the system and offer the same performance as the Firefly. The Firefly can act as a stand-alone board but also has spots for daughter cards to increase functionality.