The answer to your first question depends on who you ask. Technically, both are correct. Just don't mix and match. Pick which way you're going to do it and stick with it.
See: http://en.wikipedia.org/wiki/Serial_comma
Second Question, the correct punctuation is:
"Hi, how are you?" he asked.
And third question, the correct punctuation is:
My heart gave a wild lurch. I exclaimed, "Holy crap!" and jumped back, my heart now racing.
In both cases, you're not needing to use a capital after your dialogue because you're not starting a new sentence. Never capitalize "he said" or "he asked" after dialogue.
I think what you might be seeing that's confusing you is instances where people write something like "He laughed." This gets capitalized because it's separate from the dialogue. Characters cannot "laugh" dialogue. (Or, for that matter sigh, whimper, etc.) So it gets to be its own sentence.
no subject
See: http://en.wikipedia.org/wiki/Serial_comma
Second Question, the correct punctuation is:
"Hi, how are you?" he asked.
And third question, the correct punctuation is:
My heart gave a wild lurch. I exclaimed, "Holy crap!" and jumped back, my heart now racing.
In both cases, you're not needing to use a capital after your dialogue because you're not starting a new sentence. Never capitalize "he said" or "he asked" after dialogue.
I think what you might be seeing that's confusing you is instances where people write something like "He laughed." This gets capitalized because it's separate from the dialogue. Characters cannot "laugh" dialogue. (Or, for that matter sigh, whimper, etc.) So it gets to be its own sentence.