Aligning two divs side-by-side

If you wrapped your divs, like this:

<div id="main">
  <div id="sidebar"></div>
  <div id="page-wrap"></div>
</div>

You could use this styling:

#main { 
    width: 800px;
    margin: 0 auto;
}
#sidebar    {
    width: 200px;
    height: 400px;
    background: red;
    float: left;
}

#page-wrap  {
    width: 600px;
    background: #ffffff;
    height: 400px;
    margin-left: 200px;
}

This is a slightly different look though, so I’m not sure it’s what you’re after. This would center all 800px as a unit, not the 600px centered with the 200px on the left side. The basic approach is your sidebar floats left, but inside the main div, and the #page-wrap has the width of your sidebar as it’s left margin to move that far over.

Update based on comments: For this off-centered look, you can do this:

<div id="page-wrap">
  <div id="sidebar"></div>
</div>

With this styling:

#sidebar    {
    position: absolute;
    left: -200px;
    width: 200px;
    height: 400px;
    background: red;    
}

#page-wrap  {
    position: relative;
    width: 600px;
    background: #ffffff;
    height: 400px;
    margin: 0 auto;
}