How Much Memory Can I Have For A 2009 MacBook?
There were multiple MacBook updates in 2009, so it depends which model you own. The early 2009 models can support 6GB of 800MHz DDR2 PC2-6400 SDRAM while the late 2009 model can handle 8GB of 1066MHz DDR3 PC3-8500 SDRAM.
Learn which MacBook model you have at the App Sliced MacBook memory section.
And whenever you upgrade memory, be sure to buy a brand name model (such as Crucial, Kingston, OWC, Samsung) rather than generic. It's also a good idea to avoid buying RAM upgrades from the Apple Store since their prices are marked up considerably. Learn more and find the best prices at App Sliced Memory.