how to print submenus depending on the node id

I'm working on my custom theme based on Zen (the template not the philosophy)
and i have a menu where i want an added class on the "selected" list item. I had done something similar with wordpress which was pretty simple

<ul id="nav">
     <li id="home" ><a <?php  if ($is_front){ echo " class=\"selected\"";}?> href="index.html">HOME</a></li>
   <li id="about" class="menbutton"><a href="about.html">ABOUT</a></li>
   <li id="beauty" class="menbutton"><a href="beauty.html">BEAUTY</a></li>
  <li id="thesalon" class="menbutton"><a href="thesalon.html">THE SALON</a></li>
   <li id="thestudio" class="menbutton"><a href="lacys photography.html">LACYS PHOTOGRPHY</a></li>
    <li id="contact" class="menbutton"><a href="contact.html">CONTACT</a></li>  
 </ul>

if you look at the first list item i have the sort of if argument i want .
The one on top works perfect
but i need one where it would do it depending on the node id
this is what i have tried to do but it doesn't seem to work

<?php if ($node->id == '7'){ echo " class=\"selected\"";}?>

can anyone help me out i'm sure it's a very simple argument to code but I's just a newbie
also can you do something like this ,

<?php if argument =='7|8|45' ->the result ?>

where 7|8|45 means if it's either of these values?
I want to implement that on another part of my template to print out a sub menu
so that it would be something like this

<?php> if the node id is either 7|8|45 { print
menuhtml = theme_menu_tree(45); print $menuhtml;}
else { if the node id is either 3|6|454|67 { print
menuhtml = theme_menu_tree(5); print $menuhtml;}?>

I hope what i'm saying makes sense.
Thanks

Luis