dude still needs to be thrown some light upon. it can be used in different, but similar ways, depending upon the americans (or any english speaker who's using it).
like, "dude, when r u going home?" or "dude, where's my car?" :P here dude is more of a stress. it does not mean friend, but nor does it have a negative sense. it's just a way to address a person, usually male.
similarly, "oh go and ask that dude over there" or "hey dude, watch out". here dude may/moy not have a -ve connonation. depends on the situation. it's more "teen/youth" who find "cooler" subtitutes for "boring" words like a man or a fellow :P
chap is also a british word for fellow "hey, old chap!"