How to show entries with IF statement based on field
Your if statements looks correct. You might check that you are testing the value vs. the name (i.e. 'Portrait' vs 'portrait') or printing out the value (using {{ entry.postAspectRatio }}) so you can see the output. Also, a 'dryer' way to write this might be:

{% block content %}

    {% for entry in craft.entries.section('projects').order('postDate asc').limit(500).find() %}

        &lt;div class="title {{ entry.postAspectRatio|lower }}"&gt;
            &lt;a href="{{ entry.url }}"&gt;
                &lt;p&gt;&lt;span&gt;{{ entry.studio }}&lt;/span&gt;{{ entry.title }}&lt;/p&gt;
            &lt;/a&gt;
        &lt;/div&gt;

    {% endfor %}

{% endblock %}