Well, my solution for redundancy has always been 2 or even 3 internal HD (depending on finances) and as big as I can afford them. Also, if your mobo support it try to go for SATA disks, as they are pretty fast, but unlike dandog I do not like too much partitions, in my experience, when a HD goes, partitions don't help much.
Just my two pennies
Saludos
Just my two pennies
Saludos