I know this may not be the right or the best place to ask for WordPress support but the question is simple. Is backing up of the WP database enough to preserve all your site's content or some additional manual backup of files is required?

Zagor, great question. Please refer to this for help.

Steve has given you the best link to refer to. I can tell you that it is important to keep other back up copies. I had a friend who recently lost around 3 months of posts because of a corruption. I would not risk having just one copy if the blog is of any importance to you.

Taking back ups on your own will always help you. it keeps you confident in your work and also self dependent.

taking your own backups is the best. Make sure you back up FTP -> and MySQL db's using Export option in phpmyadmin

Thanks! I installed the plugin WP DB Backup, which can do scheduled backups by sending the database to a specified email. The only down side is that it does it weekly and cannot do monthly, which is suitable for my bogs.

I always keep a copy of my articles locally, as well as the data that goes with them, tags, categories, excerpts... I know that keeping database backups is better, but nevertheless I do this as well.

It's always best to keep a back up of your articles. And it would also be great if you store them in different locations, like in CDs or EHD. But nonetheless, there is a specific Wordpress plug-in for backing up entries. So there's no need to panic once you've installed this plug-in.

That depends on a website data how often you blog, how often you want to do this, and how you would feel if your database were lost along with a few posts. It is your decision.