It looks the width of #wrapper and #header is creating problem as you have specified width:1000px. And when you re-size browser to 800 it re-size the header also since you have specified width of wrapper and header to 1000px it creating trouble.
Why don't you change the move <div id="page-top-holder"> <div class="clear"> </div> </div>
I have been through your code, the problem is with your Wrapper width which is 1000px. Since you have given fixed width to wrapper and when you re-size the browser window to 800 resolution the header automatically get width as per the resolution and wrapper takes 1000px which forces the header to create gap.
A possible fix is to reduce the width to 772px from 1000px and adjust the other DIV widths accordingly. You might also need to change the structure of your page. Let me know if you want me to redesign it.