Recording, Live Performance, Advice??, Audio Interface, software Options

[ando]

HI,

I have just bought a PB 512mb etc...Ideally I would like to plug my guitar, vocals, 2nd guitar and hopefully 4th vocals in for live performance. Can I use the firewire 410 for this. I assume so but my real question is then if I am using program...say logic or protools can I then loop what I am playing through the computer, back to amp and then to the speakers. I am kind of confused as to what the out in the audio interfaces do....do they take the sound coming in, allow it to go through the computer program and then back along that channel to the out? Sorry if this seems to be a stupid question

I currently have a powered mixer which lets me record into garageband (which i know is great for simple things but not multichannel simultaneous editing and recording) obviously 1 channel at a time but its hopeless for intergrating my powerbook for a live gig.