GarageBand 2 can record up to 8 real tracks at once, subject to the speed of your computer and hard drive, and the capability of your audio/digital interface.
In order to record multiple inputs simultaneously, you'll need microphones sufficient for all of your physical instruments and singers. You may want to put multiple microphones on the drumset (overhead, bass drum and snare are typical). This implies the availability of a mixer as well. The guitar amplifier is usually mic'd, but sometimes recorded through an electronic amp/cabinet simulator like a Line6 Pod. You can either mic the bass amp, or take it by cable using a direct box, preamp and/ or compressor. You can rent all this kit from your local music store.
If you are recording 4 players live with mics, you will have considerable leakage between instrument/vocal mics. Not necessarily bad, but it limits your options for overdubbing or editing later.
For the audio-digital interface, there are USB and Firewire solutions to get your sounds into the Mac. For $300 and less, you generally are limited to 2 inputs. For $400-500 you can get 4 inputs (which is the practical limit of USB), and 8-input Firewire units go $800 - 2000 roughly speaking.
There is considerable variation on the features, such as how many of the interface's inputs have microphone preamps on them (as opposed to a line inpute for keyboards, for example).
Brands to look at are Edirol, M-Audio, Mackie, Terratec, Metric Halo, Presonus, Mark of the Unicorn, Steinberg, Digidesign.
First order of business though: Go to the library and read up on home recording. There are good books available, and also magazines that have frequent articles on recording technique and reviews of software and hardware: Recording, Sound on Sound, Future Music, Electronic Musician, Keyboard, Computer Music.
