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