I just replaced the original Toshiba 40GB HD in my 12" PowerBook G4 1GHz with a Seagate Momentus 2.5" 100GB HD. The computer's working great now and the drive runs quietly.
Ultra ATA/100 interface
Less than 26 acoustic dB
Installation of the hard drive was a snap and took less than an hour. I used the following guide to assist me: http://pbfixit.com/Guide/53.11.0.html
1. Place the laptop on bubblewrap for ease of handling.
2. Use a good quality Phillips/Hex set (I used a Wiha System 4 Antistatic Precision Tool Set: Ball Hex 1/16, PH 00, PH 0, PH 1).
3. Use tape to keep all screws near their holes (Guy Kuo’s advice)
4. Do not try to lift up the keyboard until you’ve removed the long black Phillips screw next to the memory card.
5. Lift up the keyboard slowly so as not to tear the foil tape used to cover the keyboard connector ribbon. Use a sharp blade to carefully peel off the foil tape and bend it back without breaking it. There are several locations where such tape must be peeled back.
6. Touch the inside of the battery bay, which is not anodized, in order to neutralize charge before working inside the chassis.
7. When removing the upper case off the chassis, try to lift the case straight up as opposed to tilting it.
8. Retighten screws carefully.
9. Make sure that you’ve unwarped the keyboard before reinstalling it. It’s better to have the keyboard bowed slightly down (upside down U) then up (right-side up U).
10. When reapplying foil tape, use the back of your fingernail to smooth it out.
11. Restart the computer after you have initialized your new hard drive before attempting to do a clean install of OS X.
PS: This thread might belong in the PowerBook forum.