The issue you are having is not related to the box model problems in IE. It's down to the way IE handles floats, in other words it doesn't do it properly. In your case the solution is easy as you have a fixed width container so you can just specify the width of the content div to make sure it never overlaps the menu. Try the following code
Code:
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<style type="text/css">
body {
margin: 0;
font: normal 12 px Arial, Helvetica, sans-serif;
text-align: center;}
#container {
width: 800px;
border: 1px solid gray;
margin: auto;
padding: 10px;
text-align: left;}
#banner {
padding: 5px;
margin-bottom: 5px;
background-color: rgb(213, 219, 225);}
#content {
padding: 10px;
border: 1px solid gray;
width: 560px;}
*html #content {
width: 570px;}
#sidebar-a {
float: right;
width: 200px;
margin: 0 0 5px 5px;
padding: 5px;
background-color: rgb(235, 235, 235);}
#footer {
clear: both;
padding: 5px;
margin-top: 5px;
background-color: rgb(213, 219, 225);}
</style>
</head>
<body>
<div id="container">
<div id="banner">Project Hugo</div>
<div id="sidebar-a">Put a list here <br> that <br> looks <br> like <br> this <br> </div>
<div id="content">ooh Content jslhdakjhd ajda aslkjd kjsadhlad dkjahs hjsdkadh test this word size cut aksdhlaskdha ajsdas dhasks asdlasd askjdhs dalkjhd askjdhd dhdjs dkjahsdlkjsahdlkjsd dhsdja</div>
<div id="footer">footer</div>
</div>
</body>
</html>