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I'm supposed to use DDR2 5300 (667MHz) RAM for my MacBookPro 4,2. The max is 6GB, and I already have 4GB (2 x 2GB), so I only need a single stick of 4GB. So far it looks like this spec RAM runs about $100.

4GB of DDR3 RAM runs about $20.

What are the consequences if I put the wrong RAM in? Will it work at all? Will it damage the RAM or the computer?

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DDR3 has a different pin and spacer configuration than DDR2. The two are not interchangeable.

If you manage to cram the DDR3 stick in, you'll really mess up your laptop. I strongly suggest you don't try it.

This image shows the difference :

(image from Wikipedia - User:Martini)

Thanks, your diagram is worth 2^10 words.

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Definitely not compatible. – Matthieu Riegler Jan 1 '13 at 12:40

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