Do you have a standard "brand name" computer??. If so, then Crucial.com, from the link below, should help you identify your RAM type(and purchase the RAM, if you wish). Just follow the procedures at the site to determine your RAM requirements. If you don't have a brand name computer, then the "Belarc Advisor" which can also be downloaded from the link, should allow you to determine what you have and need.
Hope this helps.
I have 128MB DDR currently installed (PC2100 I believe). How do I check if it is PC2100?? I know my board supports up to that speed. I want to get another 128 stick so I was wondering also if there would be any conflict with RAM from another company??