Hey, it's me again ;-). Paste it at the same end of checkout.thankyou.php. Change conversion_id and language of course ;-).
<!-- Google Code for purchase Conversion Page -->
<script language="JavaScript" type="text/javascript">
<!--
var google_conversion_id = 100000000000;
var google_conversion_language = "pl";
var google_conversion_format = "2";
var google_conversion_color = "FFFFFF";
<?php $total = $db->f("order_total"); ?>
if (<?php echo $total ?>) {
var google_conversion_value = <? echo $total ?>
}
var google_conversion_label = "purchase";
//-->
</script>
<script language="JavaScript" src="
http://www.googleadservices.com/pagead/conversion.js">
</script>
<noscript>
<img height=1 width=1 border=0
src="
http://www.googleadservices.com/pagead/conversion/1060552975/imp.gif?value=<? echo $total ?>&label=purchase&script=0">
</noscript>