The RAM sticks in each slot should match; 256/256 or 1 Gb/1 Gb. I don't know anything about your HP laptop but most new models can handle 2 Gb easily.
It is a notebook, the question is, is it a 4000 CTO? 4000LA? or 4000XX?
The notebook has one memory module compartment, located on
the bottom of the notebook.
You can increase the amount of RAM (random access memory)
in the notebook with an approved optional memory module.
Each notebook memory module slot supports one 256-MB,
512-MB, 1024-MB, or 2048-MB DDR2 SODIMM memory
Some notebook models are shipped with one of the 2 memory
module slots vacant. On other models the memory module slot
may contain a preinstalled memory module that can be replaced
at any time.