These are the books that I used to study for Boards, and I highly recommend them:
First Aid
BRS Path
BRS Phys
High Yield Anatomy, Histo, Neuro, Embryo, Psych
Med Micro Made Ridiculously Simple
Lippincott's Pharm
Lippincott's Biochem
The 3-book BSS Series (Organ system review)
Most of these books you should either already own from your preclinical coursework or you could easily find other med students who would be willing to sell used copies of their books. The only book that I had to buy new was the First Aid for the Boards (which you should get a new copy of, anyway).
Also, Step Up hadn't been published yet when I took Boards, so I don't know anything about the book.
IMO, First Aid is not sufficient as your primary book if you want to do really well on the Boards. I still read over the book about 3 times, but I only really remembered stuff from First Aid if I reviewed it after I had just studied the respective topic from my other books. Also, try to get question books as you go rather than all at once. Most people I know bought way more question books than they knew what to do with, and most of their books went unused.
Last little piece of advice -- from my experience and my classmate's experiences, try not to study too far in advance. Chances are you will not remember many details of what you studied more than one month ahead of time. It's a good idea to do some review during your current classes to get a head start, but keep in mind that you will need to study that stuff again (although you may start at a somewhat higher level of knowledge/understanding when you do study.)
Hope that helps!