The new drive will be bigger than the old, right? Seagate hard drives come with a program called "Disc Wizard" and I believe you can download it from their website too. This will let you clone the contents of your old drive to the new one.
IMO it is a good idea to format and start over fresh every once in a while. With every OS install you do something "wrong" or silly and gather some useless installed software and other cruft. Better start over, avoid the mistakes, and reinstall only the stuff you really need.