Do you know what the model of you dekstop or Laptop is?
Back to you question, either way you need to look at the memory inside you desktop or laptop ideally. This is to know how much you want to upgrade to. Say you have x2 512MBs' and you want 2GB then you would buy x2 1GB's and the x2 512MB's would be redundant. if its a laptop then you may only have say x1 256Mb or x1 512MB fitted.
Personally its safer to manually select on crucial website from Brand - Motherboard - Model or Computer - Model.
Maybe you got a dell dimension. That would be a start. post more info.