Sidebar Width

I’m using a two-column layout and I’m trying to narrow the right column (sidebar). I’m editing the following css:

#primary {
	float: left;
	width: 750px;
}

.two-columns #secondary {
	width: 400px;
}

Whatever value I add to primary, I’m subtracting from secondary (to keep the same total), yet changing these numbers “breaks” resizing the page and the margin between the main column and the sidebar. Is there another value I need to include in my css?