The way I handle this is to use JQuery to 'fix' the width so that it does not change on hover even though the bold does change. This basically takes the css width answer here and automates the process.
(function($) {
$(function() {
$('#menu > li').each(function(){
wid = $(this).width();
$(this).css('width', wid+'px');
});
});
})(jQuery);