Dude, man and guy are all very informal. I would only say "dude" with my closest friends. "Guy" is not something you would ordinarily use when addressing someone directly, rather, you would use it to refer to someone you don't know. I can tell you that I use "hombre" in Spanish when I want to say something like this. For example, I say "que tal, hombre?" when I mean "what's up, man." I think Tio in Spain is also a pretty good translation. I am not a native speaker of Spanish, however.