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Preview: what the working site looks like.

Change position of menu by changing the text-align:left/center/right; of the menus containing div. Try the following changes to move it to the right and then pad it in a bit:

ul#menu  {
    background: #FFC;
    margin: 50px 0; 
    padding: 0 20px 0 0;
    text-align: right;		
    list-style: none; 
    overflow: hidden; 
		}  

Remove the space between the buttons with a little negative margin and replace it with a different color divider:

ul#menu  {
    background: #FFC;
    margin: 50px 0;
    list-style: none;
    overflow: hidden;
    border-top: 1px solid #f00;
    border-bottom: 1px solid #f00;
}
#menu li a,
#menu li a:visited {
    color: #999;
    background: #6C9;
    margin: 0 -2px;
    padding: 5px 20px;
    border-left: 1px solid #f00;
    text-decoration: none;
}
<ul id="menu">
    <li><a href="#">a long one</a></li>
    <li><a href="#">two</a></li>
    <li><a href="#">three</a></li>
    <li>
        <a href="#"style="border-right:1px solid #f00;">
        and four is even longer still</a>
    </li>
</ul>
The markup
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Untitled Document</title>
</head>
<body>    
    <div id="container">    
        <ul id="menu">
            <li><a href="#">a long one</a></li>
            <li><a href="#">two</a></li>
            <li><a href="#">three</a></li>
            <li><a href="#">and four is even longer still</a></li>
        </ul>
    <!--container--></div>  
</body>
</html>
        
The CSS
html, body {
	margin: 0;
	font: 100% "Trebuchet MS", Arial, Helvetica, sans-serif;
	color: #000;
	background: #fc6;
}
		/* HTML5 tags */
		header, section, footer,
		aside, nav, article, figure {display: block;}
#container {
	width: 900px;
	margin: 10px auto;
	background: #666666;
	color: #fff;
	overflow: auto;
}
		ul#menu  {
			background: #FFC; /*color of menu bar*/
			margin: 50px 0; /*just to put a little space around it*/
			list-style: none; /*removes the list bullets*/
			overflow: hidden; /*hides the extra padding color on a*/
			text-align: center; /*the key to positioning the ul*/
		}
		#menu li {
			display: inline; /*in place of float:left/right; This is what makes #menu take on the display:left/center/right;*/
			line-height: 25px; /*the key to enclosing li in the ul*/
		}
		#menu li a,
		#menu li a:visited{
			color: #999; /*text color*/
			background: #6C9; /*button color*/
			padding: 5px 20px; /*padding the a adjusts the size of the button, adjust #menu li{line-height:xx} accordingly*/
			text-decoration: none; /*removes links underline*/
		}
		#menu li a:hover { /*hover effects*/
			background: #C63;
			color: #000;
		}