GarageBand ’08: Requirements for 24-bit audio
GarageBand ’08 offers support for 24-bit audio recording and playback. This requires that your audio interface supports 24-bit audio.
If you are not sure about the capabilities of your audio interface, consult its documentation, or manufacturer. If your interface does not support 24-bit audio, it's still possible to play back existing 24-bit recordings from GarageBand, but they will play at 16-bit resolution. All recordings made with 24-bit hardware will offer 16-bit resolution.