This is basic, basic, basic, but I don't know where to begin.

I've got a small AJAX form, and the tiny forms are lite, they work great, and this specific one is for an "Email to a Friend" sort of thing. However, I want to put multiple "Email to a Friend" boxes on the page, under a page with multiple listings, for a real estate site.

So all I want to do is specify where the callback alert - e.g. "Success!" or "Error. You didn't fill it out correctly" - is going to go. The code that I have now hardcodes it to a specific element Id called "c_form_target". I'm going to have multiple "c_form_target" so I want to be able to pass that parameter of where the alert goes - e.g. "c_form_target1", "c_form_target7", etc.

So this is what I came up with to pass the "target_id", using code I've already had and you can see that I'm trying to pass "target_id" as a param, first to makePOSTRequest and then further to alertContents. All I want to be able to do is call alertContents(target_id) with a param, but that won't work:

Code:
function makePOSTRequest(url, parameters, target_id) 
{
  http_request = false;
  if (window.XMLHttpRequest) { // Mozilla, Safari,...
     http_request = new XMLHttpRequest();
     if (http_request.overrideMimeType) {
     	// set type accordingly to anticipated content type
        //http_request.overrideMimeType('text/xml');
        http_request.overrideMimeType('text/html');
     }
  } else if (window.ActiveXObject) { // IE
     try {
        http_request = new ActiveXObject("Msxml2.XMLHTTP");
     } catch (e) {
        try {
           http_request = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (e) {}
     }
  }
  if (!http_request) {
     alert('Cannot create XMLHTTP instance');
     return false;
  }
  
  if(target_id == undefined){ http_request.onreadystatechange = alertContents; } else { http_request.onreadystatechange = alertContents(target_id); } //THIS IS WHAT'S NOT WORKING!
  
  http_request.open('POST', url, true);
  http_request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  http_request.setRequestHeader("Content-length", parameters.length);
  http_request.setRequestHeader("Connection", "close");
  http_request.send(parameters);
}

function alertContents(target_id) {
  if (http_request.readyState == 4) {
     if (http_request.status == 200) {
        result = http_request.responseText;
        
        if(target_id == undefined) {target_id = "c_form_target";}
        
        document.getElementById(target_id).innerHTML = result; //THIS IS WHERE THE RESULT GETS SPIT OUT TO           
     } else {
        alert('There was a problem with the request.');
     }
  }
}
I appreciate the help fellas. I know it should be basic, but it's tricky to me.