My daughter's MacBook has a miserably small hard drive, just 250GB. Being a semi-professional photographer, all those RAW files filled up the drive in no time.
I figured she should replace the hard drive, and then we came into a spare one after my daughter-in-law's HP laptop went bye-bye. The CPU was shot, but the 640GB hard drive was fine. After I got all the files off the drive and safely to my daughter-in-law, I realized I had the ideal drive for my daughter. It was ideal because the drive was (a) the right kind, (b) 400GB more capacity than her existing one, and (c) free!
The drive that your MacBook will probably use is any 2.5" SATA drive commonly used by just about any notebook computer today, Windows or Mac. You can tell if it is the right drive if (a) it is about 2.5" wide and (b) if its connector matches the one in the MacBook.
I looked into what it would take to replace the drive, and it was laughable easy:
1. Download SuperDuper! free from Shirt-Pocket Software: http://www.shirt-pocket.com/SuperDuper.
2. Attach the new drive to the MacBook. (I got an empty external drive box for $18 from the local geek computer store.) USB is better than FireWire, for some FireWire drives do not allow booting. If necessary, use the MacBook's DiskUtility software to (a) erase and then (b) format the external drive with "Mac OS Extended (Journaled)."
3. Run the software, copying all files from the MacBook to the external drive.
4. Check that the external drive will boot: restart the MacBook while holding down the Options key. You are presented with the option of which drive to boot from: pick the external one.
5. If the MacBook boots successfully, you can shut it down, remove the old drive, and insert the new one.
I won't give you instructions in how to pull the old drive, because different generations of MacBook have different ways of accessing the hard drive.
(The only hiccough was that after rebooting, Dropbox wanted me to sign in, even thought I didn't need to.)
I say it is laughable easy, because this would not be as simple with a Windows computer.