<form action="..." method="..." target="_blank">
<input type="submit" name="pagepreview" value="Preview">
</form>
If you need to change the target dynamically:
$('input[type="submit"]').click(function(event){
var $this = $(this);
var $form = $this.parents('form');
$form.attr('target',$this.attr('formtarget'));
$form.get(0).submit();
event.preventDefault();
});