Your partitioning scheme in the hard disk is the information on the screen when you type at Bash shell
The files in /etc.fstab are those the Linux must load at boot time.
If you use RAID-1 you can kiss dd and tar goodbye as you won't be needing them. Basically you put two identical disks in but only one can be seen and used. The other one just mirror the first one at the background. You have access to the second one only when you break up the RAID.
However not all Linux can understand your RAID system and some will treat them as different disks thereby upsetting the whole arrangement. I broke up my RAID because of this trouble.
DD is easy but duplicating a 80Gb disk is a 45 minutes job. A fast processor may cut it down to 25 to 30 minutes. Thus I would in your case just tar partitions out if I go down that route. In a recent tar exercise with a 5 Gb partition by Suse 10.2 x86 64 the time was 100 seconds.
You will find Mandriva a lot better to use as it is one of the most modern distros. The Mandriva I have is 4.8Gb large after installation and I would give a 10Gb space to it.
If you need to backup your work on the daily basis you need to use more sophisticated than the simple way I have suggested or go for a different command. I can't help you on this so you need to put up a new thread or do a bit of search.
For my money I would partition my drive as
/dev/hda1 in Fat32, size 2Gb and install FreeDos onto it.
/dev/hda2 in Fat32, size as large as you need for your work, say 20Gb
/dev/hda5 as Swap, 1Gb large. Being 5th means this is the first logical partition
/dev/hda6 as 5Gb large for Debian
/dev/hda7 as 10Gb for Mandriva
The hda1 allows you to access all your work without a Linux. If the MBR disappears completely you can still boot it up with a Dos floppy and check all your work
hda2 is what you need to backup. You can do it with Dos's copy command or Linux's drag and drop in Windows without bothering about dd or tar. Being smaller, at least initially, it should be easier to maintained.
I recommend the first logical partition as the swap because every Linux needs it and it helps to remember where to find it if it is always the first logical partition.
You don't need to backup Linux itself. If one goes down you can use the other to rescue it. Don't mix your data with an operating system either. You can always re-install a Linux but but not your data. You may need to mount the data partition hda2.
With the exception of the swap you can put Grub in any partition. I would use the Grub provided by either Debian or Mandriva initially and finally put it into hda2 the data partition which is accessible to Dos and two Linux
So take your new hard disks back to the shop, ask for a refund and buy back the kids now.