Or you could do this, for different banners:
PHP Code:
<SCRIPT LANGUAGE="JavaScript">
<!-- Begin
function redirectPage() {
var url640x480 = "http://www.yoururl.com/urlwithbanner640_480.html";
var url800x600 = "http://www.yoururl.com/urlwithbanner800_600.html";
var url1024x768 = "http://www.yoururl.com/urlwithbanner1024_768.html";
var url1280x1024 = "http://www.yoururl.com/urlwithbanner1280_1024.html";
if ((screen.width == 640) && (screen.height == 480))
window.location.href= url640x480;
else if ((screen.width == 800) && (screen.height == 600))
window.location.href= url800x600;
else if ((screen.width == 1024) && (screen.height == 768))
window.location.href= url1024x768;
else if ((screen.width == 1280) && (screen.height == 1024))
window.location.href= url1280x1024;
else window.location.href= url640x480;
}
// End -->
</script>
</head>
<body onload="redirectPage()">