Question about photo storage, and back-ups

I am purchasing a 24" iMac very soon (long time Mac user but " beginner" in digital photography"...

I am debating computer harddrive size. I know many use an external drive for photo, video and music instead of the computer HD...My thinking is..isn't it iBEST to have two copies of photos, etc. in the event of a HD crash? If you use only an external HD and it crashes, all is lost.... or vica versa (computer HD only).

I would think you use the computer HD for primary storage and then back-up with an external? If you use an external drive only then you use another external to BU the external? If so, how do you do that?

Maybe I am making this more complicated than it is? LOL

Thanks.
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Z812 IS owner : http://www.flickr.com/photos/13065200@N06/