In my personal view, there is definitely a difference between the LSATs from the 1990s and even early 2000s and the more recent ones (I'd say 2003/2004 to present). I don't know if I'd say one or the other is harder (for me, I fortunately found the more recent ones to be easier than the earlier ones, but I know others who find the reverse to be true), but there's definitely something different. I know it's lame to say "it's just different," but I honestly don't know how to exactly explain the difference in words; I guess the earlier ones just have a different feel than the newer ones, which I know is still vague. But does anyone else notice this too?
In any event, I would recommend definitely relying mostly on the recent exams starting with maybe 2003 or 2004 and going up to the most recent one available (is December 2007 out?), especially when you're taking timed practice tests. This I believe will give you a much better indication of what your real LSAT will look like and how you will most likely score. That'll give you about 15 practice tests to do timed in simulated LSAT conditions. As for all the other tests (2002 and back), I would just use them for practicing different question types, diagramming, marking up reading passages, and whatever other logical methods you are learning.