As a professional pilot, you are "expected"(as opposed to required) to have the most current edition. That would be the one with the current year on it.
Updates to the FAR's are issued continuously in the same manner as D(distant)NOTAMS. The Gleim or ASA FAR/AIM is just a reference. It is not an official or legal publication. Essentially these books are an excerpt of the aviation related portions of the CFR(the BIG book). These companies update these books several times a year or as necessary.
Generally the updates are so petty that if you are just using the book for studying or general reference, you won't know the difference between the 03' and 06'.
If you are flying for real though, I would suggest buying, at the very least, an 05' edition("the one with the current year on the cover").