I was getting 500 errors after an installation. After surfing some forums I learned that the permissions should be the following (magento/ being your root Magento folder).  For some reason, they weren’t necessarily correct by default:

777

• magento/var/.htaccess
• magento/app/etc
• magento/var
• all the directories under magento/media

755

• magento/
• magento/install.php
• magento/index.php

644

• all other files

Hope this helps anyone who may have been in the same situation.