I have a MacBook 2009 core 2 duo 2.4 ghz with 2 gb ram. I tried to upgrade to 4 gb, 2-2 gb sticks. I used cent on macready DDR2 800/667/533 ram. I thought this was the right ram. When I upgraded the system wouldn't boot. It will boot with old ram or with one new stick and one old stick giving me 3 gb of ram. I don't understand everything says I can upgrade to 4 gb, even some say 6 gbs. Please help. I bought a package and I cut it open, this will be a pain to return.