A JavaScript object can be of great help when you need to pass multiple values to a Modal Window. E.G.
var objArgs = new Object();
objArgs.Value1 = "Value 1";
objArgs.Value2 = 2;
var modalWindowFeatures = 'dialogHeight:600px;dialogWidth:600px;scroll:no;status:no;resizable:no';
url = 'relative/path/to/page';
var returnValue;
returnValue = window.showModalDialog(url,objArgs,modalWindowFeatures );