Also, don't forget that even if you make the box-like portions seperate, you can always select the main object and both box-like objects and choose Polygons>Combine. If you're going for a low poly model and want the model to be one single object, this would be the best way to go rather than splitting the faces up as I illistrated above.

Tip - after you extrude, you can press the g key to extrude again. Note: deselecting your face will negate this effect.

Side note - There is actually an even faster way to split the front face up than what I did above. Instead of splitting the face up horizontally first, split it vertically first. This eliminates the extra edges on the right side of your front face that you probably won't even use.

Sorry for the three post guys.....