If you go to Crucial's web site and put in the make & exact model it will give you the options available
(other major RAM vendors have similar sites & info)
we have no idea what model laptop , if it needs DDR, DDR2 or DDR3 and what speed
CPU_Z will tell you what you currently have.
If you have 1GB RAM CPU_Z will confirm it is 1 x 1GB and not 2 x 512MB sticks of RAM
If you have 1GB RAM add matching specification 2GB of RAM
If you have XP not much point in going beyond 2GB if Vista or 7 3GB will make things somewhat better.