Just another confirmation.
What you want to do is possible and advisable.
As myrkul states, "...PIE (Pre Internet Encryption) is the way to go..."
Translation. Encrypt your things before you store them online. That way you will be safe (assuming you have a strong password/pass phrase)., if that site is ever breached (from a hacker and/or rouge employee).
Also as myrkul points out. Dropbox's encryption should not be solely relied upon (well he didn't say that, however that is what I took out of it
). It they are breached, Dropbox has the keys for their encryption.
Think about it. It they are hacked, the hackers will take the data and the keys.