Yes, before you use the memory card in the SD450, you should copy the photo files to your computer.
Then burn the photos files from the computer to a CD for a back-up.
Then verify that you can read the files on the CD.
That leaves you with two good copies of your photos (one copy on the computer and one copy on the CD).
Then you should format the memory card with the SD450. That format will cause all of the photos on the memory card to disappear.
I know, a lot of people use the memory card for long time storage of photos. That is OK as long as you have a back-up copy somewhere else.
Question for SNAPSHOT. I'm replacing my canon A510 with a canon SD450. Does the m/card which has some images stored from the A510 need to be reformatted for use in the SD450? If so, will it affect the existing stored data?