Garageband Latency, Guitar Monitioring Options

[roders]

I've been playing about with Garage band & my guitar, plugging my guitar directly into my Mac (with a 1/4 Cinch jack to mini jack lead to plug in) in Audio midi setup I've changed the input (and output) sampling rate to 24Bit, but when I change the input as well as output smalping rate to 96000 Khz (to reduce latency) I get problems.
I can only set either the input or the output to this resolution (not both at the same time, which ever I set first seems to stick, and then whichever I set second doesn't) or I get a click and the resolution immediately sets back. If for some reason the resolution change does stick in Audio Midi setup. As soon as I open Garageband I hear a click, and if I check Audio Midi setup, the resolution has gone back down again.

What I want to know is, is they're a workaround in the OS? Is it a bug in the OS? Is it a limitation of Garageband? Or do I need a separate Audio interface?

P.S Garageband is set to low latency in the prefs and only one audio channel is active/monitored.

Hardware/Software versions

Macmini Core Duo 1.66Ghz 2GB ram, upgraded 7200 rpm internal laptop drive.
Mac OSX 10.5.4 Leopard, Garageband v3