New harddrive, opportunity for organization?

My old trusty Macbook has been crawling along for too long now, so I decided to rebuild it with a new battery, more RAM, and most pertinentily, a 500GB harddrive. Two other relevant variables: I have a music collection that I've been working on for decades in a disorganaized fashion, with some of it stored on my current (full) Macbook harddrive and some on a terabite external harddrive.

My questions for you amazing Mac savants:

-How do I get all my files from my current hardrive (including the operating system?) onto the new one as I swap them out? The only way I could imagine doing so is shuttling everything onto the external harddrive, and then back on to the new harddrive after it is installed..

-Since this virgin harddrive is as of yet uncluttered, I was thinking of using this transfer as an opportunity to consolodate all my music and cleanse it of duplicates. Is there away I could give a command to import all music files from the external harddrive straight into the itunes music folder on the new harddrive, but without importing any duplicates?

Thank you so much for your help! This is a puzzle far beyond my capabilities...

With much appreciation,

-Tommy