How do I install logic 9 on an older macbook pro ?

I downloaded logic 9 on my new macbook pro through to App Store. I want to also run it on my older macbook (Early 2008) currently using mac os x 10.5.8. How do I upgrade the Os X so it works and how do I install logic on the old one ?

- I can't go from 10.5 to 10.8

- I need at least 10.6.8 to run Logic Pro 9

- Once the Os X is up to date, how do I transfer the app, if possible avoiding downloading 20Gb again.