I'm in agreement with Andrew. Though in my experience, "Buddy" is usually more condescending, belittling, or sarcastic than it is lighthearted. And when it is lighthearted, it usually sounds stiff or awkward.
"Dude" is a 俗語 term. It's informal, and used between friends of both genders, though typically, boys don't call girls dude. Um, think of mixing ~ちゃん and おまえ to get "dude".
When it's used with strangers, it typically has a negative connotation, but not always.
old people don't use dude. or man.
"Man" is also 俗語。 Man is a good term when used with other people, but is sometimes used as an expression of slight disappointment "ah man."
I hope this adds some more clarity on top of what Andrew said