Esto es lo que buscas?
Código:
<html>
<head>
<title> New Document </title>
<script language="JavaScript">
<!--
var w = screen.width;
var txt = "Resolución = ";
if(w < 740){
alert(txt + '640');
}
if(w >= 740 && w < 835){
alert(txt + '800');
}
if(w >= 835){
alert(txt + '1024');
}
//-->
</script>
</head>
<body bgcolor="#FFFFFF">
</body>
<hr noshade size=1><img src="http://www.gograph.com/Images-8712/ClipArt/cat03.gif" height="50" border=0 align="absmiddle"> <font size="2" face="verdana" color="#000000">
bet[/CODE]