...that is the question.
In advertising, it is important to capture the reader's attention. With print advertising, a common technique is to CAPITALIZE THINGS THAT ARE IMPORTANT. This is not the case on the Internet. On the Internet, USING ALL CAPITAL LETTERS IS LIKE YELLING!!!! Yelling, is not selling--until the auction 
Use bold for headings.
Keep your normal text "normal" so that people feel comfortable. On the Internet and in email, capitalized text is generally perceived as "yelling" and is not as effective.