I recently purchased an iMac G5 and I am trying to decide what interface I should buy so that I can record guitar and vocals. I also plan on buying a MIDI keyboard. I am in an interesting situation because I have a great set of surround sound speakers at home that aren't being used. I am wondering if it is at all possible to hook up the speakers to the iMac or an interface. Right now the speakers (2 front, 1 center, 2 back) are connected to a broken reciever via. standard speaker cable. My questions are:

Is it possible to hook up an interface to a reciever that has surround sound capability?

Would all sounds then come out of the speakers or would sound still come out of the iMacs speakers?

This is all really new to me and I'm not sure what interface would be best for me. I've looked at the MBox (although I don't know how to use protools it seems that it would be a good thing to learn), and various M-Audio interfaces (like the USB and the Firewire 410). Any info about this would be extremely helpful, thanks!