TV tuner cards tend to have buffers of up to 2 seconds, making them completely unsuitable for game consoles. However, if you persist, what you'll need to do is go to your local Radio Shack and get an RCA to 1/8" adapter. This then plugs into the Line-In jack on your sound card.

Just don't act surprised if you find out that the video and audio are way out of sync, and there's a delay between the time you hit a button on the controller and you see the resulting action on screen.

The best method is not to attempt this with a computer unless you have a monitor that supports S-Video and/or Composite inputs such as Dell's LCD line 20" or above. Then use some sort of receiver unit to handle the audio end of things, such as Creative's DDTS-100, or any other home theater receiver unit. Of course you could easily get a new TV for far less than that.