is it possible to add a class to header found in page.tpl based on a node id?
You probably already have a specific class for the page on the &lt;body&gt; tag.

For example :

&lt;body class="html not-front not-logged-in page-node page-node- page-node-5 node-type-page"&gt;


Then a css should work for you :

body.page-node-5 #main-content-header { ... }


or 

body.node-type-page #main-content-header { ... }