If you want it for music and videos then you would want a higher capacity
one. Find out how much space your media is using (Right click on the folder
where you have your music, videos, photos, etc. and select properties.) and
get a HD thats a much bigger than that if you plan on getting a lot more
music, videos, etc.
If you want it for backups then you might do fine with one thats the same
size as your internal one (right click and select properties on the local
disk (C
and it will tell you the capacity). There are some that include
built-in backup software.
If you want it as a portable HD to store music, files, etc then get one that
doesn't need a power cable or get a HD mp3 player that can act as an external
HD (Archos, Cowon).
Go to
www.newegg.com for reviews on HDs.