Memory stick, copy to another computer, burn it to a CD. Put it on a second hard drive, all of the above.
I've got backups going back to 02, including terrains, entire settings folder, sights folder, etc.
Settings folder should only be about 1.2 MB or so, similar for Sights unless you have loads.
Terrains will take up more space, but they are optional.
With the low cost of SD or SDHC cards and USB adapters or readers you could even backup to an SD card.
Some of the current crop of SD cards are shipping with an adapter that looks like a usb stick, only it has a slot in the other end for SD card.