Does your Onkyo amplifier accept digital SPDIF input? I have my mac mini attached to my Onkyo receiver, which has two optical SPDIF inputs, which is in turn hooked up to my 7.1 surround sound system. All Apple computers have a digital out port (which is part of the traditional headphone jack). All you need to do is get a mini-TOSLink to TOSLink adapter cable, which can be had on the cheap at monoprice.com or other places. Then, your computer can send the digital output to the receiver, which decodes it and sends the signal to your speakers.
Of course, if your amplifier doesn't support digital inputs, I would suggest getting a USB or FireWire breakout box card, as has been suggested. You can then attach your analogue connectors from the breakout box to the amplifier and then have your sound.