You have to also change the following width in the template_css.css file
#topnav {template_css.css (line 25)
background:#585C68 url(../images/topnav-bg.gif) repeat-x scroll left bottom;
font-size:12px;
margin-left:auto;
margin-right:auto;
padding:0pt 10px 10px;
position:relative;
text-align:left;
width:740px !important;
}
Try to search for the width of relevant element in the template and decrease the number.
Just wonder why you want to change the width to 600px, 740px is a safe width for all modern browsers, and I think more than 60% internet user are using 1024×678 or greater.