Following Christian's suggestions I've amended my code and added a 'current' class. This is working fine (and probably cuts down on DB calls):
{# set values for the navigation #}
{# products = structure #}
{% set productpages = craft.entries.section('productPages').depth(1).find() %}
{# singles #}
{% set contact = craft.entries.section('contact').first() %}
{% set overjanandries = craft.entries.section('overJanAndries').first() %}
{% set overview = craft.entries.section('overview').first() %}
{% set news = craft.entries.section('nieuwsIndex').first() %}
{% set section = craft.request.getSegment(1) %}
<div id="menu">
<a class="close"> <span class="menu-icon icon-cancel"></span></a>
<nav role="navigation">
<ul>
<li {% if section == overview.uri %}class="current"{% endif %}>{{ overview.getLink() }}</li>
<li {% if section == overjanandries.uri %}class="current"{% endif %}>{{ overjanandries.getLink() }}</li>
{% for page in productpages %}
<li {% if section == page.uri %}class="current"{% endif %}>{{ page.getLink() }}</li>
{% endfor %}
<li {% if section == news.uri %}class="current"{% endif %}>{{ news.getLink() }}</li>
<li {% if section == contact.uri %}class="current"{% endif %}>{{ contact.getLink() }}</li>
</ul>
</nav>
</div><!-- menu -->