If you already have a decent Graphics card, then I'd just add a TV tuner card.
If you were thinking of upgrading it anyway, then ATI normally have an "All in Wonder" model for most of their ranges, while Nvidia based cards have the "Personal Cinema" - though that's the posh version with a remote.
On the other hand, the "all your eggs in one basket" approach, means that if you want to upgrade the graphics again, you have to pay the extra again, while a basic TV tuner/capture card can be quite cheap - more expensive models may have better software (PVR - timed recording) or a hardware encoder so that it can record direct to DVD-compatible MPEG.
To just watch TV, one of the cheaper TV cards will do.