E finalmente Ajax:
Codice PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Our wonderful AJAX page!</title>
<script language="javascript" type="text/javascript">
function DOMize(element) 
{
    if(!element.addEventListener)
    {
        element.addEventListener=function(event,listener)
                                {
                                    event="on"+event;
                                    if(this.attachEvent)
                                    {    
                                        this.attachEvent(event,listener);
                                    }
                                    else 
                                    {            
                                        this[event]=listener;
                                    }
                                 }

    };
    if(!element.removeEventListener)
    {
        element.removeEventListener=function(event,listener)
                                    {
                                        event="on"+event;
                                        if(this.detachEvent)    
                                        {
                                            this.detachEvent(event,listener);
                                        }
                                        else     
                                        {        
                                                delete this[event];
                                        }
                                    }
    };
    return element;
};
function getPage(){

var xmlhttp=false; //Clear our fetching variable
        try {
                xmlhttp = new ActiveXObject('Msxml2.XMLHTTP'); //Try the first kind of active x object…
        } catch (e) {
                try {
                        xmlhttp = new
                        ActiveXObject('Microsoft.XMLHTTP'); //Try the second kind of active x object
            } catch (e) {
                xmlhttp = false;
                        }
        }
        if (!xmlhttp && typeof XMLHttpRequest!='undefined') {
                xmlhttp = new XMLHttpRequest(); //If we were able to get a working active x object, start an XMLHttpRequest
        }
        var file = 'query.php?page=';
         //This is the path to the file we just finished making *
        var sel = document.getElementsByTagName("select")[0];
        var whatinfo = sel.options[sel.selectedIndex].value;
        xmlhttp.open('GET', file + whatinfo, true); //Open the file through GET, and add the page we want to retrieve as a GET variable **
        xmlhttp.onreadystatechange=function() {
        if (xmlhttp.readyState==4) { //Check if it is ready to recieve data
                var content = xmlhttp.responseText; //The content data which has been retrieved ***
                if( content ){ //Make sure there is something in the content variable

            if(test = document.getElementById("paragraph")) {
            document.getElementById("target").removeChild(test);
            }
                var info = document.createElement("p");
                info.setAttribute("id", "paragraph");
                var text = document.createTextNode(content);
                info.appendChild(text);
                document.getElementById("target").appendChild(info);
            }
        }
        }
        xmlhttp.send(null) //Nullify the XMLHttpRequest
return;
}
window.onload = function()
{
var sel = document.getElementsByTagName("select")[0];
DOMize(sel).addEventListener("change",getPage, false); 
}

</script>
</head>
<body>
<form method="get">
<select name="choosepage">
<option  value="">Choose page</option>
<option  value="uno">Page One</option>
<option value="due">Page Two</option>
</select>
</form>
<div id="target"></div>
</body>
</html> 
Mi sembra che inizi ad essere decente come implementazione
come si dice da cosa nasce cosa basta iniziare .......
un pezzetto di quà un'altro di là