I was in luck, the 640GB Western Digital Scorpio was on sale so I made the purchase while preparing for the Super Bowl last Sunday.  Tuesday afternoon rolled around and I already had my hard drive so I began this amazingly easy process.  I will break it down in steps below:

Step 1: Hardware

Follow Apple’s instructions on how to physically remove the old and input the new here http://manuals.info.apple.com/en/MacBook_13inch_HardDrive_DIY.pdf

NOTE: You need a really small Philips Head screwdriver to take out the Ram door.

Step 2: Software

If you are like me and have Time Machine set up, plug it in and insert the disk of the operating system you are using (I cannot stress this enough, using a Leopard disk to restore a Snow Leopard Time Machine will NOT work).  Go to Utilities and select ‘Restore from Time Machine.’

Step 3: Wait

It normally takes between two and three hours for the backup to complete, so go play Modern Warfare 2 or something like I did.

Step 4: Restart

And you’re done.  Time Machine backs up your system so well that it even saved my browser history and Firefox opened to the exact page I was on. Amazing.