#left_side {
    width: 635px;
    display: block;
    float: left;
}

#left_side:after {
   content: ".";
   display: block;
   height: 0px;
   clear: both;
   visibility: hidden;
   float: none;
}

#right_side {
    width: 300px;
    display: block;
    float: left;
    margin-left: 5px;
}

#right_side:after {
   content: ".";
   display: block;
   height: 0px;
   clear: both;
   visibility: hidden;
   float: none;
}