I would install the new HD and do a clean install. Computers always run so fast on a clean install.
Then plug the old HD into a free sata port, leave it dangling if you dont have a bay, or if it is a laptop I would spend 10 bucks on a Sata to USB adapter.
Then just copy all the stuff you need from the old HD to the new.
You will still have to reinstall all applications and software though.