I think you mean no driver with 10.5.7, Jeremy, since the driver is meant to work in conjunction with the operating system, not the audio recording software. There wouldn't be a driver specific to Garageband '09.
But you're right, the solution probably is in the Audio Input preferences.
Though I don't have one, I have heard that the line/guitar input on the Fast Track is sort of low. If you don't have a way to boost the guitar's signal using some sort of preamp (and a guitar overdrive pedal would be sufficient for that) you could use the overdrive effect in GB to boost it. Just set the Gain low and the Output high on the Overdrive and it will give you a more or less clean boost of the recorded audio on that guitar track. You could also use something like FreeG, which is a free plugin that you can use to adjust the volume of any track. You can get that here:
http://www.sonalksis.com/index.php?section_id=99