Pretty much what btmiller said.
If you're doing SW RAID, you need to fdisk the partitions you want to use and set them up as type fd and use mdadm to construct; see http://www.linuxhomenetworking.com/w...et_The_ID_Type
You cannot 'use' the extended partition, as its not a real one as you'll see by looking at the Start/End figures. Its just a placeholder/container for the logical partitions.
Also, there's little point in using less than an entire disk (still need to partition as one big partition).
The usual use of RAID is to protect you in some form from disks going bad.
Note this does NOT constitute a backup.