My iMac 20-inch early 2008 uses 800MHz memory. (Has 2GB) My dead (logicboard) MacBook Pro early 2008 uses 667MHz memory. (Has 4GB) I've replaced the iMac 2GB(800MHz) ram with 4GB (667MHz) from the macbook pro into the iMac and it seems to be running fine. Can I expect any problems from this change is MHz? What's the difference between 800MHz & 667MHz memory? What is the limit of MHz memory I can put in the iMac? Is 2GB(800MHz) faster/better than 4GB(667MHz)?