Thank you all very much for your help.

Would it be wise to backup the entire data directory, in addition to wallet.dat?  It seems like that would save me having to re-download the older blocks if I lost my installation.

I have a laptop, which I use to do all my bitcoin-relating activities.  If my laptop were to get stolen, or if the HDD died, I would lose all of my bitcoins, wouldn't I?  Is there a way I can prevent this?