To me, it looks like the problem is with the <div id="search_box">. It seems the property "left: 775px;" on that <div> is causing the large space to the right of the site because you are pushing the search box all the way over to make it look as if it's floated to the right of the page. I looked at the site with Chrome Developer Tools and applied a background color of red to that <div> and that's where I saw it hanging all the way out to the right.
To fix this, you could try reworking that area by placing two <div>'s within the <div id="stripe1">, and float one to the left and the other to the right, while aligning text accordingly.
<p>A paragraph here...</p>
<input type="text" name="search_box" />
<input type="submit" name="submitBtn" value="Search" />
<div style="clear: both;"></div>
margin: 0 auto;
You may have to do some adjustments to paddings and margins to get everything aligned cleanly, but hopefully that helps.