All of the changes from R14 on are much better for speed, but there are definite changes in the way Acad looks at entities. If you have any lisp routines, there is a good chance that some of them will no longer work. If you decide to make the change, let me know, if there aren't too many programs, I will update them for you, gratis.
2000 is almost the same as R14 with some real nice features added and some real nasty glitches possible. Most of the glitches have been corrected now (Version 2000i).
2002, from what I have heard, is a problem child. Best bet would be to wait for 2003 if you don't get 2000i.
All in all, If I had all of the possible updates between R12 and 2002, I would install in this order:
R14 or Map3
Map4
2000i
And if there was room I would keep all three. Set 2000i to save all drawings in the R14 format. Then save all R14 drawings that are simple wblocks in R12 format. The savings of 20+KB per drawing will make drawings smaller. Who needs a dictionary in every drawing?