On latency:
It's a non-issue. OS X was designed for audio latency in the single-digit milliseconds, comparable to dedicated music hardware devices like synths and hard disk recorders. The big problem on older Macs was the OS (specifically, the Sound Manager part of the OS), not the hardware. You can now install OS X on a four year-old tangerine iMac and play it like a big distortion pedal -- in real time. D-Sound's GT Player application works well for this, as does IK Multimedia's Amplitube Live. Garage Band should work similarly in this regard.
On plugging in:
If you buy the 1/4"-to-1/8" Radio Shack adapter and plug straight into the analog audio port on your Mac, the volume will be low. The Mac's audio-in jack was designed for low-impedance, line-level signals, not high-impedance, instrument-level signals, so your guitar output will have only barely enough juice. This will be fine for most things, but dirty tones, like amp simulators on overdrive, will not be as overdriven. The impedance mismatch plays a role in this as well. If the new Monster Cable that Apple is marketing for use with Garage Band has an impedance matching transformer, this would likely improve this situation. Worth a try.