What your visitors see on their monitors is critical to how they respond. If your site/page design hides or keeps important content outside the eye view of a visitor then you probably have wasted that visitor.
Google Labs has a free tool where you enter any url and presto Google displays the site with an overlay showing what visitors will see by monitor display setting as well as percentage of visitors.
click for larger image
http://browsersize.googlelabs.com/
Good article on Google Browser Size Tool