For the best sound, you should get a usb sound card and send the digital to an external d/a converter.
This one is cheap. http://www.turtlebeach.com/site/products/audioadv/micro/
This one is expensive, but probably worth it if you have the bucks. http://www.m-audio.com/products/en_us/AudiophileUSB-main.html
There are also usb d/a converters that do a great job.
The worst sound will come from taking the analog out of a PCI sound card or your motherboard's onboard audio.
This one is cheap. http://www.turtlebeach.com/site/products/audioadv/micro/
This one is expensive, but probably worth it if you have the bucks. http://www.m-audio.com/products/en_us/AudiophileUSB-main.html
There are also usb d/a converters that do a great job.
The worst sound will come from taking the analog out of a PCI sound card or your motherboard's onboard audio.