I just replaced the HDD in my wife's macbook - performance was slowing to a crawl, she wouldn't let me buy her a new one. So I went here:
powerbookmedic.com
If you put in the serial number it will tell you what works - don't worry about what was, get whatever is cost-effective. They have instructions on how to install.
Picked up a refurb 120gb drive for <$40; five minute hardware install, Mac found the drive, New OS on and back up and running quickly.