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The word act, as used in the term Act of Congress, is a common, not a proper noun. The capitalization of the word act is deprecated by some dictionaries and usage authorities. Some writers, in particular the U.S. Code, capitalize Act.
I believe that the capitalization of a title depends on the medium. In a formal medium such as an academic paper, the first word and all words other than articles (a, the, etc.) and prepositions (of, under, about, etc.) should be capitalized. However, in more casual situations (such as web logs and Q&A sites), only the first letter must be capitalized.