When I was studying for this past July Part 1, I used the latest edition (2001) of the Dental Decks. My classmate who I studied a lot with had the 1998 edition of the Dental Decks. Although many will tell you there's no difference, but personally I find that the 2001 edition is much more thorough, concise, easier to read, and more information on the back of the flashcard. There are many distinctive differences.
My classmate and I actually compared flashcards on the exactly same topics, my 2001 edition was much more better than his 1998 edition because of better explanations and more details, tables, and pictures. After comparing many flashcards between the two editions, we've found that there were much more information on the 2001 edition and we both agreed that 2001 were much better.
Further, in the 1998 edition, the information/question in the front of some the flashcards were very plain and vague while the 2001 edition had all multiple choice questions on them in the front. Again, the 2001 edition was much more defined and effective.
Further yet, 1998 edition has a red background on the back of all flashcards while the 2001 edition has a white background where you can highlight all the main points for faster reviewing of materials. Another feature that many of my classmates found useful.
Are they the same? No they're not.
Is one better than the other? Yes, 2001 is much better!
Is it absolutely crucial to get the 2001 edition to do well in Part 1? No, it isn't!
Side notes:
1) NBDE Part 1 format is slowly changing. For example, this past July exam in the Dental Anatomy section had no picture questions at all while in the past exams you see at least 5-10 picture questions. Later this year, Part 1 will be computerized and as early as next year, Part 1 will be a completely different format (complete comprehensive patient based clinical questions, similar to USMLE)
2) New edition of the Dental Decks will be available some time in 2004 (the 2004 edition). New Dental Decks editions are released every 3 years because of the release schedule of ASDA past NBDE Part 1 released exams.
Good luck on your studies!