One major exterior change is that there are plastic bumpers on 89-91 and not on 84-88 (except for late in 1988). I am unaware of there being better headlights, but that depends on your personal definition of 'better'.
If you are comparing two different 325's, one with plastic bumers, one without, I personally would recommend getting the one with the plastic bumpers (if the cars are very similar in options and condition). I am going to change the bumpers on my car to plastic bumpers once I can afford them, because they look much better then the metal ones (in MY opinion).
<img src='http://home.comcast.net/~illuzi0n/bmw-sig.jpg' border='0' alt='user posted image' />
<a href='http://www.cardomain.com/member_pages/view_page.pl?page_id=439310&cat=25&val=1' target='_blank'>This is my cardomain site</a>