ciappati questo:
inserisci questo script

<script>
function apripopup() {

var windowprops='width=1,height=1,scrollbars=yes,statu s=no,resizable=yes,location=no';
var heightspeed = 6;
var widthspeed = 6;
l=(screen.width-700)/2;
t=(screen.height-600)/2;
var leftdist = l;
var topdist = t;
var lunghezza = 700;
var altezza = 600;

if (window.resizeTo && navigator.userAgent.indexOf("Opera")==-1 && navigator.appName.indexOf("Netscape")==-1 ){
var winwidth = lunghezza;
var winheight = altezza;
var sizer = window.open("","titolopopup","left=" + leftdist + ",top=" + topdist +","+ windowprops);

for (sizeheight = 1; sizeheight < winheight; sizeheight += heightspeed)
sizer.resizeTo("1", sizeheight);
for (sizewidth = 1; sizewidth < winwidth; sizewidth += widthspeed)
sizer.resizeTo(sizewidth, sizeheight);

}
else
window.open('nomepopup.php','',700,600);
}
</script>

e poi nel form:
<form action='nomepopup.php' name=prova method=post target="titolopopup" onsubmit="apripopup()">