Because many sellers can’t give the following information (they don’t know or they don’ t want you to know and give erroneous info):
I own a MacBook Pro 3.1, e.g. Santa Rosa chipset, processor T7400, 2,4 Ghz. I replaced the Seagate Momentus 160 Gb 7200 rpm by a Seagate Hybrid drive Momentus ST750LX003. I closed the jumper to restrict the drive to Sata1 transfer, and it works terrific!
Moreover, I replaced 1 of the 2 Corsair MacMemory SODIMMS by a 4Gb PC5300 module, type CT51264AC667. Though the remaining Corsair Module is a CL4 and the Crucial one is CL5, this works perfectly together. So I can now use the full 6 Gb, which enabled to install an Ubuntu 13 VM with 3Gb storage space.
So. it’s absolutely not necessary to buy a 6 Gb set from OWC or from heaven knows where.
In Europe, the Crucial SODIMM can be obtained for as low as € 79,- if you do some Googling.