On each of your buttons you could have the following;
<input type="button" name="newWin" onclick="frmSubmitSameWin();">
<input type="button" name="SameWin" onclick="frmSubmitNewWin();">
Then have a few small js functions;
<script type="text/javascript">
function frmSubmitSameWin() {
form.target = '';
form.submit();
}
function frmSubmitNewWin() {
form.target = '_blank';
form.submit();
}
</script>
That should do the trick.