Bicycle Quarterly Magazine tested a number of tires for rolling resistance a couple of years ago. The old issues are not on the web for free so you have to dig up an old hard copy. The surprising result was that width is not the only determinent for rolling resistence. Some wider tires were faster than narrow tires in roll down tests. On their web site you can find some interesting observations by Jan Heine on tire width; www.bicyclequaterly.com.

Whether wide tires are faster or not, they are more comfortable. Right now I have Panaracer Col de la Vie 650B (684x38) on a Univega conversion, and find that they absorb road shocks great. New England roads are not much smoother than dirt now.