Your HEADPHONE output is analogue. If you want to take an audio signal from your computer through your HEADPHONE jack, get one that splits into two channels (stereo) and then plug one into an L and one into a R input on your receiver or pre-amp.
If you want to take the signal out of your computer in digital, you can take it from the USB or FIREWIRE port.
If you take it from your HEADPHONE jack, then your computer has already done the DIGITAL TO ANALOGUE CONVERSION using a cheap DAC (DIGITAL TO ANALOGUE CONVERTER) in the "dirty" environment inside your computer.
If you take the signal out of your USB or FIREWIRE port, you will get an extremely clean high quality signal and now you can do your D to A conversion cleanly, outside your computer with a higher quality DAC. Depending on how much you want to spend, you can get significant improvement over using the headphone jack all the way up to a digital front end that will compete with CD players costing thousands of dollars.