@patdavid mentioned this, but I thought I’d link to the whole article: http://www.dpbestflow.org/backup/backup-overview
I think that article provides a very nice baseline for keeping your data backed up.
I would hesitate to call files in Dropbox, especially if the Dropbox is being synched everywhere, a backup. Dropbox has lots of redundancy on their end and certainly it is another copy of your files, but it is not a backup. What if you accidently remove all the files and don’t notice? Not that this isn’t aimed at Dropbox specifically, but any solution that is automatically syncing your files.