I bought my back up drive from buy.com - very happy with their service and they are just off the 73.
Here is the link to their externals. I'm fond of the
2.5 inch, as they are easier to carry around. I have the 120 gig version of
this (and paid the same price a few years ago). The 160 gig is $3 cheaper.
Whatever you decide to go with, make sure you don't need an external power source (most are powered off the USB cable these days).