Copy 1 is copied from code supplied. Main content is 490px wide. It is calculated like:
780px (i.e. main containner) - 250px (i.e. side bar) - ( 20px+20px - i.e. margins on both sides) = 490px
Copy 2 Main content is modified to 450px by adding 20px of padding to each side of the div.
Copy 3 Sidebar width is now 300px and the width of the main content section is 80%. The margin is removed from the main content div.
Copy 4 Added red italic text and a light orange background on the header and footer. It is more like copy 3.