HJW is correct.
I will respectfully dissent on the recommendation though.
I say get your RAM from a vendor who tests and guarantees the RAM in your model Mac, offers a lifetime warranty and a no-charge return if it doesn't work. (Which would not be one of the PC Generic discount places)
Some members like Crucial (despite their generally higher prices), OWC, RamJet. If you are in the USA I like Data Memory Systems
http://www.datamem.com
Thanks
Trevor
CanadaRAM.com