I use a pair of external USB drives to do a regular backup, though not as often as I really should
You can buy a drive bay like this:
http://www.aria.co.uk/Products/Compo...roductId=30522
and a HDD like this:
http://www.aria.co.uk/SuperSpecials/...roductId=30998
and put them together yourself for some fairly massive storage. Or, buy a brick with the bits already put together like:
http://www.aria.co.uk/SuperSpecials/...roductId=28945
Freecom do some highly regarded external drives, though I've found the supplied backup software less than perfect.
Duncan