PC setup- different audio to different outputs?

Is it possible to set it up, for example, where I can listen to the line in from my turntable through computer speakers and game through my headset?

If the game lets you choose your audio output device then yes, otherwise no. Playing back audio from your line in will go to your "default playback device" as set by the Sound options in the Control Panel. If the game you are playing only plays to the default playback device and doesn't let you choose the audio device from within the game, then probably not. I'm not aware of any way to direct Windows to play one source on one output and another source on another.

-Byshop