One thing to try in case you haven't before, Blonderella, is when using a volumetric fog, turn down the Volume Quality while preview rendering. Then when you're ready to render, turn it back up to 50 or so... Might look funny in preview mode, but it will preview much, much faster and allow you to actually develop your scenes still. I have plenty of really cool fogs and cloud materials, if you want any to play with, let me know and I'll email them to you. If you're not certain what the hell I'm talking about, I'll email your a quick sample project file to mess with!