TV tuners are generally for use in conjunction with a Media Center OS, or Tuner box which allows you to stream signals from your set to your PC. If I am understanding your question correctly then I think the sufficient Video Card is what you are after.
Depending on your PC's video card, you can hook your TV up to your PC and use it as a monitor. If you have a video card with an S-Video out, and the ability to run Dual-Monitors from the video card itself, then that is all you should need.
The only downside to using a TV as a second monitor is that unlike monitors, TV's use pixels in a different way, and the refresh rates differ. This may matter less the farther up the video card ladder you go. Also, I hooked a 36 in set to my PC, and it wasn't too bad, and I am guessing that the graphic quality may be a bit better on a smaller TV. But I don't know that for a fact.
So basically, you use a TV Tuner to watch TV on your PC, and you use your Video Card on your PC to view your PC on your TV.(assuming you have a compatible Vid Card).
If I may ask, what kind of video card do you currently have installed on your system? Also, you need to make sure that if you do not already have the sufficient card, that your system will support added hardware slots.
Hope this helps,