Honestly speaking...laptops aren't upgraded, they are replaced, IMO (I don't own one).
Laptops are made with the intent of selling newer models. Due to basics of their design, there isn't a lot that can be upgraded and, as time goes by, those options become fewer with newer models coming forth.
IMO, it would be a waste of money to upgrade anything on a system that old...unless it's strictly a case of monetary contstraints. If that's the case then I would suggest considering a simple netbook from the typical offerings of today.
Max RAM is double what you have installed, approximate cost $50, .http://www.crucial.com/store/listparts.aspx?model=Inspiron%208100%20Series
You could put a larger hard drive in, cost about $60, http://www.newegg.com/Product/Product.aspx...N82E16822116040
After you do these things...the system will still be exceptionally slow by today's standards, if working. The fact that you have such an old laptop...means that any key component may fail at any moment and you would then spend more to have it repaired than to buy a much better system.
New system: http://www.walmart.com/browse/Computers/La...elected_items=+