Don't plug the turntable directly into your sound card. The signal coming out of a turntable has to be re-eq'd by a phono pre-amp. I know, stupid stuff, but that's how vinyl works. If you don't do this, it won't sound right.
So, more ideally, you'd connect your pre-amp/receiver's audio out (tape loop perhaps) to your sound card. You'll also need a special cable that has 2 RCA plugs on one end and a 1/8" stereo mini-plug on the other.