Centrer mon menu horizontalement

Bonjour,

Cela fait 2 jours que je galère ...
Je n'arrive pas à centrer mon menu horizontalement sur ma page, je voudrais également le limiter à 820px éventuellement.
Merci de vos réponses.
Voici mon CSS :

.bg {background: url(images/button4.gif);}
.menu {padding:0 0 0 32px; margin:0; list-style:none; height:40px; background:#fff url(images/button1a.gif) repeat-x; position:relative; font-family:arial, verdana, sans-serif; }
.menu li.top {display:block; float:left; position:relative;}
.menu li a.top_link {display:block; float:left; height:40px; line-height:33px; color:#bbb; text-decoration:none; font-size:11px; font-weight:bold; padding:0 0 0 12px; cursor:pointer;}
.menu li a.top_link span {float:left; font-weight:bold; display:block; padding:0 24px 0 12px; height:40px;}
.menu li a.top_link span.down {float:left; display:block; padding:0 12px 0 12px; height:40px; background:url(images/down.gif) no-repeat right top;}
.menu li a.top_link:hover {color:#000; background: url(images/button4.gif) no-repeat;}
.menu li a.top_link:hover span {background:url(images/button4.gif) no-repeat right top;}
.menu li a.top_link:hover span.down {background:url(images/button4a.gif) no-repeat right top;}

.menu li:hover > a.top_link {color:#000; background: url(images/button4.gif) no-repeat;}
.menu li:hover > a.top_link span {background:url(images/button4.gif) no-repeat right top;}
.menu li:hover > a.top_link span.down {background:url(images/button4a.gif) no-repeat right top;}

.menu table {border-collapse:collapse; width:0; height:0; position:absolute; top:0; left:0;}

.menu a:hover {visibility:visible;}
.menu li:hover {position:relative; z-index:200;}

.menu ul,
.menu :hover ul ul,
.menu :hover ul :hover ul ul,
.menu :hover ul :hover ul :hover ul ul,
.menu :hover ul :hover ul :hover ul :hover ul ul {position:absolute; left:-9999px; top:-9999px; width:0; height:0; margin:0; padding:0; list-style:none;}

.menu :hover ul.sub {left:2px; top:40px; right:2px; background: #fff; padding:3px 0; border:1px solid #999999; white-space:nowrap; width:200px; height:auto;}
.menu :hover ul.sub li {display:block; height:20px; position:relative; float:left; width:250px;}
.menu :hover ul.sub li a {font-weight:normal;display:block; font-size:11px; height:20px; width:192px; line-height:20px; text-indent:5px; color:#000; text-decoration:none; border:3px solid #fff; border-width:0 0 0 3px;}
.menu :hover ul.sub li a.fly {background:#fff url(images/arrow.gif) 80px 7px no-repeat;}
.menu :hover ul.sub li a:hover {background:#999999; color:#fff;}
.menu :hover ul.sub li a.fly:hover {background:#999999 url(images/arrow_over.gif) 80px 7px no-repeat; color:#fff;}
.menu :hover ul li:hover > a.fly {background:#999999 url(images/arrow_over.gif) 80px 7px no-repeat; color:#fff;}

.menu :hover ul :hover ul,
.menu :hover ul :hover ul :hover ul,
.menu :hover ul :hover ul :hover ul :hover ul,
.menu :hover ul :hover ul :hover ul :hover ul :hover ul
{left:90px; top:-4px; background: #fff; padding:3px 0; border:1px solid 999999; white-space:nowrap; width:93px; z-index:200; height:auto;}


et mon menu html :
<style media="all" type="text/css">@import "/menu/menu_style.css";</style>
<div id="content">
<ul class="menu">
<li class="top"><a href="accueil.html" target="_self" class="top_link"><span>Accueil</span></a>
</li>
<li class="top"><a href="administratif.html" target="_self" class="top_link"><span>Administratif</span></a>
</li>
<li class="top"><a href="audiotypie.html" target="_self" class="top_link"><span>Transcription audio</span></a>
</li>
<li class="top"><a href="particuliers.html" target="_self" class="top_link"><span>Prestations aux particuliers</span></a>
</li>
<li class="top"><a href="avantages.html" target="_self" class="top_link"><span>Pourquoi sous-traiter</span></a>
</li>
<li class="top"><a href="clients.html" target="_self" class="top_link"><span>Références clients</span></a>
</li>
<li class="top"><a href="contact.html" target="_self" class="top_link"><span>Contact</span></a>
</li>
</ul>
</div>

<!--fin menu -->