Well, if your house burns down or someone breaks in and pinches your PC then all the RAIDs in the world are not going to help. Or if a powersurge or virus fries the whole lot.
Unless your system is really mission critical and has to be up all the time RAID is a waste of an expensive drive and not physically secure. If you need that sort of security you'd also have an offsite mirror or two.
Why not use the first 80GB drive as your working drive and periodically clone it to the second (I use Pixielabs XXCOPY
http://www.xxcopy.com/ which is free for personal use and which can do a quick incremental clone after the first long one), which you store somewhere else (I use a safe in the concrete shed at the far end of the garden). A removable HD drive rack is not expensive and you can set up a weekly reminder that is hard to bypass. That way if your drive or O/S goes Tango Uniform you can just change over and off you go.
It's a minor chore, like most safety, but there's nothing to beat that feeling of being back up and mostly intact in a few minutes.
You can even use VolumeID from
http://www.sysinternals.com/ntw2k/utilities.shtml to give both volumes the same volume number so that they will be effectively identical logically as well as physically (the serial numbers won't be the same, but many disks don't have one and I don't know of any software or OS that goes as far as to look for this -
http://www.winsim.com/diskid32/diskid32.html).
I also regularly clone My Documents, vital downloads and other essentials to ANOTHER removable drive that lives in another safe at work AND to a separate partion on my kid's XP over the network.
OK, I'm paranoid.....but I'd be interested in what a pro like williamsg says.
[Edited to correct doubledouble postingposting - what was that little PPRuNe hiccup?]