Depends on what is already in there...
if you have already got all slot occupied then you'll have to remove some to make room to fit larger capacity RAM..
otherwise just plug in the extra.
There is a scanner here that will tell you excatly what you have in your machine and what to buy, you don't have to buy from the site but it will ensure you get the correct memory
http://www.crucial.com/systemscanner/
Also don't go too mad... windows will not use more than 3.5gb of ram and to be honest much more than about 2gb is normally not worth it.