How can I use tar to backup all the essential files:
such as config files and any other important files.

and also keep out the junk that isn't system critical like man pages and docs

you can use tar in many different ways but something like this will work

mkdir /backup


tar czv /etc/*.conf > /backup/etc_conf.tar.gz

this would create a zipped tarball named etc_conf.tar.gz in /backup/ containing all files in etc that end with .conf


or use the more generic method

tar cfvz /backup/etc_conf.tar.gz /etc/*.conf

I use mondo-rescue to back up to bootable cd's, from which I can do a full system restore including programs I've installed to a blank hard drive.

are the only files i should really backup *.conf files or are their others I should backup?

That's hard to say, I only backup files I have created, files I want to save, and config files that are edited after the standard install to setup my machine.

Some people backup their whole system.

I just used the *.conf for an example. I do not backup those because only a few of them are changed in my system.

I am using Raid 1 with ext3 file system and have an UPS that will shutdown the machine if the batteries go low so I am not all that worried about loosing stuff.

One thing I do is if I have a file that I use to setup something, which I have several. I e-mail them to a couple of my addresses and then I can easily retrieve them whenever I need them again.

If you have a firewall file save it. Also if you are running a dhcp server and you have a big dhcpd.conf file you want to save it. Also zone files if you have setup a custom DNS server. mail config files for mail server, etc....