3 hard disks
After learning the hard way a few times over the years by losing data I now have 3 hard disks in my main desktop plus 3 in another. (I dual boot on one by the way with XP and Vista, mainly just to occasionally remind myself how awful Vista is!)
I use synctoy to regularly back up my important files to the other hard disks on this desktop.
Every 2 months or so I back them up to an external hard disk and then transfer them to the 3 hard disks on the other desktop.
Any really important new files are immediately transferred by flashdrive or network to the other computer.
There are probably huge flaws in my methods but for the last few years it seems to have worked despite complete system crashes.
Internal drives really are quite inexpensive these days; and I have tended to have a number of reasonable sized drives left over from upgrades - they might not be as fast as the newer ones but they still hold data reliably.
Regards
Exeng