I am not an expert in this field, but I think (somebody correct me if this is in error):
-the control panel is not part of your VPS. The control panel runs on DreamHosts webserver where they run their own website, forum and such
-how much does it take to run apache? not much… it is when you start adding Wordpress, certain plugins, joomla, web-apps, owncloud, etc that things start taking up all that RAM.
so, a minimal website with some html pages would probably use a fraction of your memory, while a WordPress site with 20 plugins installed and running WooCommerce would use almost all of the 300mb…and more if a lot of traffic!