The Crucial web site will scan your computer and tell you what memory you have, how many memory slots, and the options for upgrading.
Go to this website and select "scan my system"
http://www.crucial.com/
Make a note of the memory type it says you have and the memory options for upgrading.
You dont have to buy Crucial memory, there are loads of makes.
Note there are LOTS of different types of memory so make sure you write down exactly what type of memory the web site says you have.
After you have done the scan you can put the details here and we will help you decide your best options.