The code below is from the fixed width 2 column layout and I've highlighted the main ones that you'll most likely need to check out and change. Hopefully that will get you started if you do decide to change to the fixed 2 column layout. Good luck on it either way
<style type="text/css" media="screen">
html, body { margin: 0; padding: 0; }
body { color: #333; font: 12px Helvetica, Arial, sans-serif; line-height: 18px; }
h2 { color: #333; }
a { color: #337810; }
p { margin: 0 0 18px; }
#container {
width: 760px; margin: 0 auto; }
(this is the main site width)
/* Header */
#header { background: #eee; border-bottom: 2px solid #ddd; }
#header h1 { color: #004b6f; margin: 0 0 3px; padding: 24px 18px 0; }
#header p { color: #666; font-size: 11px; font-weight: bold; padding: 0 18px; }
/* Content Style */
#navigation { border-bottom: 1px solid #ccc; margin-left: 18px; }
#navigation ul { padding: 0 18px 9px; }
#extra { }
#extra small { font-size: 11px; line-height: 18px; }
#content { border-bottom: 1px solid #ccc; margin-bottom: 18px; }
#content p, #extra p { padding-right: 18px; }
/* Content Positioning and Size */
#navigation { float: left;
width: 175px; }
#content { float: right;
width: 540px; }
#extra { float: right;
width: 540px; } /* Footer */
#footer { background: #333; border-bottom: 2px solid #999; clear: both; }
#footer a { color: #eee; }
#footer p { color: #ccc; margin: 0; padding: 0 18px 10px; }
#footer ul { border-bottom: 1px solid #555; list-style: none; margin: 0 18px 6px; padding: 10px 0 6px; }
#footer li { display: inline; font-size: 11px; font-weight: bold; padding-right: 5px; }
</style>