Multi Track Recording with a USB Mixer/Garage Band/Macbook Pro

I am currently hosting and producing a podcast and we have built up a studio... the last thing I would like to do is to wire it up for multi track recording which will make my post production life a lot easier to preserve the show.

I am using a Behringer X1204 USB mixer which I currently run my 4 Microphones through, my seperate skype computer, my 360 Systems Instant Replay 2 (for sound effects) and my laptop which runs the BlogTalkRadio broadcast suite and my iTunes for any music or audio I may need to play.

What do I need to do in order to get each channel into Garage band so I can do the final master mix down for the show? I do a live mix obviously but sometimes I notice people may talk over a guest or caller and I'd like to be able to raise that back independently in post.

I am looking at the M-Audio Fast Track Ultra with 4 XLR inputs but I am not sure if that will be enough to run everything through it. I am using Dynamic Mics so Phantom Power isn't such an issue at the moment.

I am also uncertain as to how the device would play into my mixer. I run a live internet broadcast so I still need my behringer x1204 to handle my live mix of the show so the listeners get a polished show live, is there a way to run my mics from the M-Audio back to the mixer and then maybe to a seperate computer for the live broadcast?

Currently I am recording into GB on the Same computer that handles the iTunes and BlogTalkRadio Broadcast platform, so would I need to multi track record on one computer and then broadcast/itunes from a different one?