Looking at the design here are my first thoughts:
The right and center coulms have the same background color, so you could enclose them in a parent div, set the bg color and float one column against the other as you prefer, probably like so:
CODE
<div id="wrapper" style="background-color:silver;float:left;">
<div id="blocks" style="float:left;">
<div>
<h1>Latest Forum Posts:</h1>
....
</div>
<div class='block'>
<p>Would you like to buy a hamburger?</p>
</div>
<div class='block'>
<p>Perhaps a vowel?</p>
</div>
</div> <!-- end Blocks (right col)-->
<div id="content">
<p> Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Phasellus in mauris a ipsum placerat pretium. Phasellus pellentesque neque tempus neque convallis congue. Ut venenatis placerat mauris. Etiam nec dolor non justo placerat condimentum. Sed eu elit ut nunc blandit hendrerit. Sed in dolor. Donec dolor turpis, hendrerit a, pretium sit amet, laoreet sit amet, arcu. Duis pharetra ante vel nisi. Proin tellus tortor, porttitor ut, consectetuer eu, venenatis ac, nibh. Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Mauris nunc.</p>...
</div>
</div><!-- end wrapper (right wrapper col)-->
<div id="ads">
Ads will float to the left and make the page bg color the same dark gray as this col and the footer, define wrappers bg color as the lighter one.
</div>