And I have to chip in with my standard advice.
I recommend you look for a reputable dealer who tests and guarantees compatibility with your model Mac, offers a lifetime warranty, and a no cost (no restocking, no shipping) return if it doesn't work.
USA vendors that MR readers most often recommend are Crucial (use the link above so MR makes a couple of cents),
OWC (MacSales) and
Data Memory Systems
Thanks
Trevor
CanadaRAM.com