Call Customer Name in Header

[killertofu]

I am trying to mimic a "Hi, [name]" at the top of my page. I'll explain...

When a guest comes to my site, I would like it to say:
"Hi, Guest. Please register or login."
The "register or login" linked to the login/registration page.

Then when someone logs in, I would like it to change to:
"Hi, [First Name]."

I suck at php, but this is what I constructed so far. Don't laugh:

Hi,

<?php if ($_SESSION['customer_id']) { ?>
    <?php echo $cust->fields['customers_firstname']; ?>
    <li><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGOFF; ?></a></li>
    <li><a href="<?php echo zen_href_link(FILENAME_ACCOUNT, '', 'SSL'); ?>"><?php echo HEADER_TITLE_MY_ACCOUNT; ?></a></li>
<?php
      } else {
        if (STORE_STATUS == '0') {
?>
    <li><a href="<?php echo zen_href_link(FILENAME_LOGIN, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGIN; ?></a></li>
<?php } } ?>