Newegg has a great site to find the correct and most compatible memory for you
Here.
To maximize your ram speed, use the same speed and types of ram
For example, if you have a 1GB right now, add three more 1GB than putting in a 2GB and 1GB.
If you have a 4GB ram stick right now, add another 4GB and not 2x2GB. Also make sure the MHz on the ram is the same, (1333, 1600, 2000)
Enjoy upgrading your computer!