probleme mit div

<div class="main">
    <div class="links">
         <div><img src="gfx/side_top_left.jpg" style="border: 0px;" /></div>
         <div><a href="index.php?site=news"><img src="gfx/news_unselected.jpg" style="border: 0px;" /></a></div>
         <div><a href="index.php?site=guestbook"><img src="gfx/guestbook_unselected.jpg" style="border: 0px;" /></a></div>
         <div><a href="index.php?site=forum"><img src="gfx/forum_unselected.jpg" style="border: 0px;" /></a></div>
         <div><a href="index.php?site=imprint"><img src="gfx/imprint_unselected.jpg" style="border: 0px;" /></a></div>
         <div><img src="gfx/side_main_left.jpg" style="border: 0px;" /></div>

    </div>
    <div class="mitte">
    <div class="content-mitte"></div>
    <div><?
    if(!isset($site)) $site="news";
    $invalide = array('/','/\/',':','.');
    $site = str_replace($invalide,' ',$site);
    if(!file_exists($site.".php")) $site = "news";
    include($site.".php");
    ?></div>

    </div>
    <div class="rechts">
        <div><img src="gfx/side_top_right.jpg" style="border: 0px;" /></div>
        <div><img src="gfx/side_main_right.jpg" style="border: 0px;" /></div>
    </div>

.main {
text-align:center;
background-image:url(gfx/background.jpg);
background-repeat:repeat-y;
}
.content-mitte{
    background-image:url(gfx/middle_top.jpg);
    width:600px;
    height:30px;
    background-repeat:no-repeat;
}
.links {
float:left;
border:0px;
}
.mitte{
float:left;

}
.rechts {
float:left;
}