There were definitely some important improvements during the 986 maturation from 01 to 04. Some of the more noted were: In 2003 where the rear plastic window on the soft-top was converted finally to glass w/defrost and of course the addition of the "911 borrowed" VarioCam (variable intake valve/timing system), also I believe the base wheel size was increased to 17 (base) and 18 (S) - Then you got a real glovebox and sightly reduced height overall. With the Variocam and slight mods, I think it helped our fuel economy and slight ticks on the ET/0-60 charts, but nothing massive. If choosing though, I would definitely get a 2003 or later and certainly opt for the S model if you can swing it.
I think the only thing significant for 2004 was the Anniversary Edition but I could be wrong....I think they were holding out the rest of the massive changes for 05 when the "new model" 987s were introduced. Performance between a 02 S and a 04 S is not going to be anywhere near night and day, but as always mentioned get the latest Box you can afford in the best shape.
BTW, make sure you opt for a PPI (Pre-Purchase Inspection) completed at the nearest Porsche dealer before putting any money down. It should cost around $200-250 but it certainly can put your worries to rest at for the major items. It is not perfect but a darn good starting point. Also, there is a "national" network on Porsche.com of the dealerships offering CPO "certified pre-owned" and can definitely be worth their additional cost as most all come with fabulous extended warranties. Here is the link if you want to check it out and search for your neck of the woods....
http://www.porsche.com/usa/dialogue/contactandinformation/usedcars/
Hope this helps and welcome to the forum!
M