Here’s a link to an an excellent article about which portable hard drive to buy which appeared in PCWorld.
Watch for sales over the Christmas holidays and during New Year’s specials of hard drives with older technology.
If you’re using hard drives to backup your images (and you should) speed isn’t a big issue. A stand 2.0 USB one-terabyte hard drive will likely sell for anywhere between $60 to $99 on sale.
Last year I bought two 1.5-terabyte drives for $99 each at Future Shop. So far this year I’ve used half a terabyte so I’m ahead of the game :)
BTW if you are doing video editing or lots of photo editing with multiple editing programs running at the simultaneously then you’ll want at least one firewire, USB 3 or Thunderbolt drive (shockingly fast) and if there’s a choice in performance buy the faster drive. It will cost more but a faster drive saves time if you’re using it as a scratch pad in your digital darkroom.