There is hardware that the Police departments use for this, but they
admit only so much can be done. Just look at the news media of ATM
footage and you will see what they can't do with a pic. I have found
gimp works well when enlarging a photo and recommend it (my only photo
program). Part of the problem when working with digital cameras is not
having the RAW data that is converted to compressed formats. If your
camera can supply RAW data then you have more chance of a quality
enlargement. If you rely on jpg format set your camera to least
compression. PNG is a better format, but if you will take your pics as
a very fine compression in jpg then you can convert to PNG in gimp
before zooming in. I have been able to view at 800% and understand
what I am looking at, but that is a far cry from zooming in on a face
and telling who it is. As was stated earlier, you can only work with
what is there. 8MP cameras are cheap now days so upgrade on that front.