Hey man, you've joined SDN, just that puts you WAY ahead of most of your classmates. You certainly do NOT need to start studying yet for boards. If you really, really wanted to, you could always start with one of the detail-oriented (wait, isn't everything detail-oriented!) subjects like immuno, micro, anatomy, etc. Nothing intense. Just read for understanding, review, and recognition. Then start doing questions from the several question banks availabe (Board Simulator Series, IV Qbank, etc) or question books (Kaplan's Qbook, Appleton & Lange, NMS, etc) to test knowledge. Then towards the end of your studying, use "regular" Qbank since it's still the best question bank available.
From my research, no, there is not a huge difference b/w different editions of FA, especially if we're talking a difference of a year (2005 vs. 2006). The new 2006 edition just came out. It hasn't hit the stores yet but should be availabe at least by next week, if not sooner. Get it, use it alongside class, with other relevant review books (assuming you don't do this already).
There are some EXCELLENT advice from this years test-takers. Read them! Check out the stickies at the top of the forum or do a search for names like BigFrank, LongDong, Idiopathic, p53, HamOnWholeWheat, Omarsalleh, etc (there are many other super-stars).
Oh yeah, don't forget to join the 2006 Step 1 team (Go Cobras!). G'luck.