Note that there will no box be drawn around the text even if the function name suggests this.
After the box has been drawn you can get the new x and y position with
<?php
$x=ps_get_value($ps,'textx',NULL);
$y=ps_get_value($ps,'texty',NULL);
?>
textx points to the end of the last character written by ps_show_boxed and texty points to the baseline of the last line written (which means, if there is e.g. a 'g' in the last line then the lower part's y-coordinates of the g will be lower than the value of texty. I hope you understand what I meant)
