Just be aware that your laptop can only take so much memory. So if it says you can handle 1 Gb, then you either have to just add 512 memory module to the empty slot next to it. Now if it says it can handle 2GB, then buy the 1 Gb module if you can afford it. Reason is if you buy 512 and then later decide to upgrade to 1 Gb you lose the money on the 512 memory module. So better to buy 1 Gb then replace the other 512 Mb with a 1 Gb when you can afford it.
But this is if the Max memory is 2 GB.
Like someone said, go to crucial.com and they can get you straight on what you need.
Also, study the two below sites on the memory modules, but cnet gots the better site cuz they have the video. But both are good.
READ THIS FIRST
http://help.cnet.com/laptops/9602-12576_39-0.html?messageID=2505400&tag=tip-2505400&kw=laptops
URL with PICTURES TO INSTALL LAPTOP MEMORY
http://www.computermemoryupgrade.net/installing-laptop-memory.html
These two sites to describe memory and more bs. You gotta look
http://www.cnet.com/
http://www.pcworld.com/
RUN THIS TO SEE WHAT YOU HAVE INSIDE
INFORMATION SITES
http://www.belarc.com/free_download.html -- info on computer inside
http://www.crucial.com/index.asp -- info and price on memory