Here's a site which lists all the Army medals and their order of precedence.
They should be arranged in ascending order, from bottom to top and from left to right as you view them. That is, the least "valuable" medal or ribbon should occupy the first position on the left of the bottom row of ribbons and the others arranged sequentially from there. Only 3 ribbons to a row, then you start another one. If a subsequent row contains less than 3 ribbons, they are centered.
The Combat Infantryman's Badge (the blue badge with a musket) is positioned on top of everything else.
http://www.americanwarlibrary.com/display/usa.htm