I think the rule of thumb (from someone who has done exactly this) is that if your bike originally took 26 x 2.0, you can safely go down to 26 x 1.5, and any smaller than that you're kind of pushing it. I remember someone here giving me that answer and pointing me to Sheldon Brown's site to a chart with compatible tire sizes, but I'll be damned if I can find it now.
Anyway, I went and got a set of 26 x 1.6 Conti Sport Contacts and was totally in love with them. Best tires I ever owned. Now that I have the Raleigh coming, I can't even use them and I only put about 30 miles on 'em :-(