There are about a billion reasons to create a new category page, so I won't go into the why or what-for. Suffice it to say, you may want one so here is how to do it.
Copy the file
Pick the category display page you'd like to use as your starting point. For this document, let's assume it's CategoryGrid2.aspx. Let's assume your new file name will be "CustomGrid2.aspx". Avoid using new category page filenames of "CategoryGrid?.aspx" as future AC7 updates might include additions that overwrite your page.
Copy the file ~/CategoryGrid2.aspx to "CustomGrid2.aspx". Now you have to edit the new file so AC7 will know to treat it as a category display page.
Modifying the file
Category display pages are identified in the system using this comment line in the beginning of the .aspx file:
<Name>Category Grid (Deep Item Display)</Name>
<Description>The defualt category page that displays products in a grid format. All sub-products in all sub-categories of the category are displayed. Allows customers to browse your catalog.</Description>
In your new file copy, be sure to change the name and description values of the comment section as shown above to differentiate yours from the included pages.
You now have a new choice in the drop downs for category display pages. If you've modified the comment section correctly at the beginning of the new file, AC7 will recognize it and use it just like any other category display page.