1) verify maximum RAM per channel supported (1GB, 2GB?)
2) Do you have 32-bit or 64-bit OS? (32-bit supports roughly up to 3GB but you can have 4GB installed. it just won't use it all.)
If you have 2GB of RAM installed I probably wouldn't bother upgrading.
Usually RAM is installed in laptops in dual-channel. If you upgrade you usually have to throw out the current RAM. For example, if you have two slots only and have 2GB you likely have 1GB per slot. So you'd upgrade to 4GB at 2GB per slot (if supported). But if you have 32-bit OS you only go from 2GB to roughly 3GB this way.
If you have 64-bit Windows AND your laptop supports 2GB per slot then you can get the full 4GB of RAM installed.
*Again, in general I'd leave things alone if you have 2GB installed.