http://owl.english.purdue.edu/handou...r/g_comma.html
5. Use commas to separate three or more words, phrases, or clauses written in a series.
The Constitution establishes the legislative, executive, and judicial branches of government.
The candidate promised to lower taxes, protect the environment, reduce crime, and end unemployment.
The prosecutor argued that the defendant, who was at the scene of the crime, who had a strong revenge motive, and who had access to the murder weapon, was guilty of homicide.
Technically most English teachers stress that you should use the second comma.
*
*note: Although now that I've looked around, both are correct.