what size hard drive

Hi,

At the moment I save all photos and videos to my pc's hard drive. I print most photos out and save the videos to dvd. However I know dvds are not the best method of storage and the hard drive on my pc could fail.

I want to back them up to a portable hard drive that I could keep away from the pc. What size portable hard drive would I need. I have approx 6k photos and various videos an i this the best way to ensure I never lose the photos/videos?
thanks