The shopping arrayed on Fifth and Madison Avenues from 57th to 79th Streets is not for the faint of heart-or the faint of wallet.  Some of the most expensive stores in the world are found here, and an afternoon shopping along this strip is bound to yield some celebrity sightings.  It's an enjoyable window-shopping stroll even for those of us who can't foot the bill; the architecture and design of these stores is top-notch.