Here is how I extend my content section when I hide column2 on my page...

page.tpl.php


<div id='content' class='<?php if ($column2) print 'narrowcontent'; else print 'widecontent'; ?>'>

This says, if I have a column2 follow the css "narrowcontent" and if I do not go with "widecontent".


In Css you would have...

.narrowcontent { width: 300px; }
.widecontent { width: 700px; }

note: make sure #content has no width set in it.