Be aware that not all tires labeled 26 inch are the same. MTB 26" tires are a different bead seat diameter than 26"x1 3/8" 3-speed tires. See

http://sheldonbrown.com/tire-sizing.html

A rim of a given width can take a range of tire width, but it has limits. You can measure the inner rim width and use Sheldon Brown's rule of thumb:

"A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width."

He also has a chart on that page or rim width vs tire width.