try
a {
display: block;
padding: 10px 30px;
}
edit
Do you want something like this ? http://jsfiddle.net/Y8Ng7/
Just remove that ridiculous margin you have for the nav and increase the li padding
li {
display:inline;
padding: 10px 40px;
}
To center a div element, don't do margin: 21px 646px 21px 646px;
just do margin: 21px auto;