how old is your computer, what model is it?
to find out exactly what model it is go here: http://www.crucial.com/
go to step 1 and choose your manufacturer, on step 2 under "Not sure what you need? No problem." click on Crucial System Scanner, and it should tell you what kind model computer you got.
Then after to agreeing to a form you'll come to a new page saying "Get Maximum Performance From Your *computed model name will be filled in here* Series By Adding More Memory!"
And thats how you find out what model you have, if you post that i'll tell you if you have an agp, pci, or pci-express slot (only the newest computers have pci-express slots).