Note: I am shortening the hyper-polysyllabic C2D MBP to "MeBook" (short for MeromBook) because my fingers are starting to hurt.
Although the new MeBook ships with Mac OS 10.4.8 installed it is a different build (8N1037) than the build that is installed on my previous MBP (8L2127). For this reason you need to use the install DVDs that come with the MeBook and upgrade the OS on the donor HDD (still in the original MBP) before the swap. Allow about 30 minutes for this process and use the time to take apart the receiving MBP.
Swapping hard drives in the MacBook Pro is not a trivial task nor for the weak of heart. The difficulty level falls somewhere between the super-easy consumer MacBook, which only requires the removal of three screws, and the nightmare that is installing a HDD in any iBook. That said, it isn't impossible either.
I completed the double HDD swap (which requires opening both MBPs) in just over an hour and you should budget your time appropriately. Start by printing the Fixit Guide for the MBP from iFixIt.com. Then gather all the tools you'll need and an ice cube tray to store the screws.
You'll need a fairly large table or workbench because you're taking apart two machines and you need to keep the parts separate. I label each MBP with a small post-it because once open they're almost indistinguishable. My last piece of advice is to stay seated the entire time (especially in the dry winter) to avoid electrostatic discharge (ESD) and take your time.
Once the HDD is transplanted into the recipient you'll be greeted with the comforting startup chime of your new MeBook. Good luck!