Centering Primary Menu

I would say I am intermediate css level. I’ve tried a few things, but it doesnt do what i’m wanting. Any help is highly appreciated.
Here’s css for menu in motion theme.

.primary_menu {
width: 980px;
background: url(images/blacktrans-menu.png);
}

.primary_menu li {
float: left;
line-height: 1.5em;
margin: 5px 0 5px 5px;
background-color: #333333;

}

It helps for centering drop down sub items, rather than left alignment. The main menu buttons are what I’m trying to center. Just want equal spacing left/right of main menu items instead of having whatever space is left over depending on the names of buttons. Of course the more text and buttons I add it will fill the menu line and eventually will spill over to next if too many for the line. So even if I just have one button in main menu it should be dead center rather than all the way left.
Thanks so much. Maybe this helps clarify what I’m after.

I am reading this is solution a few places from your links. But its not working. Perhaps you can help? Thanks so much for feedback.
display: table;
margin: 0 auto;

Menu CSS:
.primary_menu {
width: 980px;
background: url(images/blacktrans.png);
}
.primary_menu li {
float: left;
background: url(images/blacktrans.png);
line-height: 2em;
margin: 5px 0 5px 5px;
}
.primary_menu li:hover {
background: url(images/blacktrans2.png);
}
.primary_menu li a {
display: block;
margin: 0;
font-size: 0.8em;
color: #fff;
padding: 0 13px;
}
.primary_menu li a:hover {
color: #afdaff;
text-decoration: none;
display: block;
}
.primary_menu li ul {
position: absolute;
width: 180px;
left: -999em;
padding-top: 5px;
margin: 0;
}
.primary_menu li:hover ul,.primary_menu li.sfhover ul {
left: auto;
}
.primary_menu li li {
padding: 0;
margin: 0;
width: 180px;
line-height: 1.5em;
}
.primary_menu li li a {
color: #fff;
margin: 0;
padding: 7px 13px;
background: url(images/blacktrans3.png);
}
.primary_menu li li ul li {
margin-left: 13px;
}
.primary_menu li li a:hover {
color: #afdaff;
}
.primary_menu li:hover,.primary_menu li.sfhover {
position:static; /* prevents IE7 drop-down menu bug (focus on a page element prevents nested menus from disappearing) */
}

Seriously? This WP theme I’m trying to center the menu on has been DLed 327,631 times. I can’t be the only one that wants this. Thanks anyway for trying.