digital connection from computer to pre-amp

i want to deliver a top quality digital signal for uncompressed music, probably from the usb port of an applemac to my primare sound processor/pre-amp. this has optical and electrical digital inputs; also RS-232. i want to use the dac within the sound processor as this has several useful surround sound modes as well as 2 channel stereo. i plan to buy a second-hand applemac (i have my eye on a powerbook) to dedicate to the task. can anyone suggest what would be the best way to link the computer and the sound processor?
Try the HagUSB (by Hagerman Technology), a small, inexpensive unit that connects the USB port on your Mac to the digital input on your processor. I believe you can get an RCA-out version, if needed.
Apart from what Sammie suggests, if you want to spend some money, I would first try Rhanson's suggestion. The HagUSB is excellent and yes you can choose between an XLR or RCA out and it costs about a 100 bucks
thanks for the advice. i have now ordered a HagUSB.