I've bought and returned 4Gb RAM that met the spec for my 2.4Ghz core 2 duo MacBook - they all should have worked but only one module would work with one of my original modules.
It was really frustrating but in the end this part number KTA-MB667K2/4G from Kingston works! I've no idea why this works and not the other standard RAM I tried!
Now I'm just pleased my MacBook is finally running with 4Gb installed!