simultaneous 8 track recording?

ok, so the newest version of garageband has been out for a while, and i am well aware that it does have the potential to record 8 audio tracks simultaneously...

however, since there is only one audio input into most computers (i have a powerbook g4, older 15 inch model), how can one go about actually recording 8 separately mixable audio tracks at once?

i experimented with one usb audio interface that allowed two tracks to be recorded at once, but i was not satisfied with it and returned it after i bought it. still, i have not seen any interface that allows for more than 2 tracks to be recorded at once. i just don't understand that if apple produces software with a specific capability, how is it possible that there is no hardware available that can be used to take advantage of garageband's functionality?

does anyone know of how to be able to actually record 8 track simultaneously with garageband? or does anyone know how to atleast record 5 at once (let's say for example, one input for guitar, one for bass, one for electric drum set, one for a midi keyboard, and one for vocals...)?