If you ask me, I'd say DEFINITELY get the 2006 version, especially if your school teaches using a systems-based curriculum. I bought the 2005 edition when it first came out and used it alongside 1st year classes. I didn't write too much into it b/c I knew I'd be getting the 2006 version during 2nd year (plus, I wrote A LOT into my BRS Path which I didn't plan on updating). Also, the 2006 version has corrected some minor mistakes found in the older versions of FA as well as "beef-up" certain sections. As far as I've noticed, only a few diagrams have been removed yet ~40 more pages has been added.
Moral of the story: Stop being cheap and get the new one! G'luck