NIS2005 can be/is very problematic to 98/SE machines. I believe that many who continue to use Symantec, updating each year, never update any system elements such as their processor, RAM and/or HD size. I currently use NIS2005 in a 98SE self-built machine, Celeron 500Mhz, 512MB RAM and 2 160GB HD's. At present I have no problems and never have with that machine and Symantec products, although now it is used as a secondary machine.
You asked another poster in this thread if there is any significant difference between NIS03-05 in resources, the short and simple answer is YES. NIS05 has added 2 new categories I can think of right now, spam control and outbreak warnings. It's up to you to try it if you wish, but I would suggest at the very most the Trial version first to see if you have issues. I would also suggest that you use a registry monitoring program to take a snapshot of before/after installing NIS05 so you can do a thorough cleanup if warranted. The best suggestion is to use an imaging application such as Norton Ghost to create an image before installing, then if all goes haywire you can restore the image to before the install.