Interface and Cobweb in some ways fall outside of the 'Stephenson timeline' I think. They were co-wrote with someone else (his uncle, if I remember what he said at the talk I saw correctly). I believe that Interface was written between Snow Crash and Diamond Age, and Cobweb was written after Diamond Age. Not positive on that though. I enjoyed both of these. They were much more along the lines of Cryptonomicon than Snow Crash or Diamond age. The Science fiction elements of them are only slight, if they are there at all. They fit more into the thriller/mystery genre for the most part. Both pretty cool. If you want to read earlier Stephenson though, pick up Zodiac, which came out before Snow Crash, and is still in print. Also very fun, especially if you know Boston, where it is set.
BTW- anyone know of a library in Boston which has the Big U? I'm starting to look around for it, but hey, any shortcuts would be appreciated.