Even though my new MacBook still had plenty of free hard drive space, I wanted to replace it with a faster (7200 RPM) model to boost the performance a bit. Since I'm also running Windows 7 in a Parallels Desktop VM, any extra storage space would help, too.
I chose a Western Digital Scorpio Black. They're pretty inexpensive ($80 at Newegg), so I went for the 320 GB size. Note that there are two versions of this drive: the WD3200BEKT without WD's "Free-Fall Sensor," and the WD3200BJKT which includes the sensor. The sensor is just a feature that parks the heads if the drive senses an impending shock. It's a great idea, but it conflicts with the Mac's built-in Sudden Motion Sensor, which does the same thing, so if you're buying for a Mac, make sure you get the WD3200BEKT.
Instructions for replacing the hard drive are included in the MacBook's owner's manual, believe it or not. Also, believe it or not, it's really simple to do. (Kudos to Apple!) You'll need a Phillips 0000 screwdriver for the case screws, and a Torx T6 driver to transfer the mounting screws from the OEM hard drive to the new one.1, 2 Note that all the screws are held in place with Loctite Blue, so you're likely to strip them unless you use the right size driver. I didn't bother to clean the old Loctite off and replace it; we'll see if I have any problems with the screws working loose.
The new drive doesn't run any hotter or sound any louder, but I do feel a very slight increase in the vibrations coming from the drive. I haven't done any formal tests, but the MacBook does boot faster and, in general, it just seems a bit snappier. Yea!
1) I read quite a few posts elsewhere that people were using anything from Phillips 0 to 000, but with my Sears Craftsman Precision Screwdrivers I had to use a 0000 to fully seat the bit into the screw's head. Almost everyone agreed, though, on the T6. I think this again shows how Torx are generally a better fastener.
2) I thought I owned all of Sears' small screwdrivers, but apparently not! I already had the 10-piece set, but there are 3- and 5-piece sets that expand on the 10-piece (with only one screwdriver in each smaller pack overlapping what's in the 10-piece). The 3-piece has both the T6 and the 0000 Phillips you need for this project.