There are 5 different design styles you can choose for your post archives with Montana regarding the number of sidebars in a page and their position. You also have control over the listing of your posts and decide whether to show them in a grid view with columns or a straight-forward time-line view.
Each post and page also have its own custom layout options with custom heading background. You can also hide these fancy headings if you want though, and use another Custom Title for your post or Page.