I would definately go out and get an audio interface. I never trust the little mic input on a computer. Getting a device that is specifically made to do the a/d conversion is the better way to go. You can get these for fairly cheap. If you are just using one mic, all you need is 1 input. Getting a bigger interface with more inputs may be something to think about if you plan on recording more in the future.
The set-up would be at2020->xlr cable->preamp->xlr cable->interface->usb or firewire(depending on what type of interface)-computer. You can probably avoid the interface if you had to but I think your overall sound will be poor. Also purchasing an interface may give you better software than garageband. Nothing against garageband, it's a decent program especially being free, but there is better out there. It comes down to preference.