What are the consequences of using DDR3 RAM in DDR2 MBP4.2? 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?
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 : 



Thanks, your diagram is worth 2^10 words.