Success! Fifth time was the charm! I tried to move up from the original 2x1Gb RAM configuration in my MacBook 6.4 Ghz Core 2 Duo to 6 Gb RAM (1x4 Gb + 1x2 Gb, all from iFixit). Followed instructions exactly after grounding myself several times etc, put the 4Gb module on the left and 2 Gb on the right but got the dreaded "three beep." Tried re-installing w/ the 4Gb module on the right--no joy. Tried re-installing old 1+1; no problem, so obviously I was seating the modules correctly. Fourth time: Just installed the new 2 GB and left on of the old 1 Gb--success! The 4 Gb module seemed to be the problem. Tried one more time--attempt number FIVE. Put the 4 Gb back in for the third time (in place of one of the old 1 Gb of course--SUCCESS. I heard a different click this time too. Anyway--I now have six times the RAM I originally had installed--Yea! Thanks, iFixit! Now, I'll work in my iMac!