Read Sheldon Brown's article on Tire Sizing: http://sheldonbrown.com/tire-sizing.html

Tire sizing is confusing. What you're really looking for is the E.T.R.T.O. size of the tire, usually listed as something like 37-590. That would be a 37 mm wide tire with a 590 mm BSD.

As long as the BSD matches, you're fine. You can mix and match width of tires and rims to a certain point.