If it's really just an HTPC and not a gaming maching, then stick with either the Nvidia 8500-8600 series, or the ATI 2400-2600 series. The 8800 series Nvida and 2900 series ATI cards are overkill. Noise and quality of video output are key for an HTPC card, not gaming power.
All of these cards include the capability to decode hi-def signals in the GPU, altough the cards in the lower end of the range (8500 and 2400) might not have the raw power to actually pull the decoding off cleanly. I think a fanless 8600GT is the best of the bunch right now. Make sure to look for (and buy) a DDR3 memory card, since the DDR2 cards are slower and not much cheaper.
As mentioned above, get a separate TV card. Most TV cards are combo cards now, and they can receive both hi-def and analog signals. The WinTV HVR-1600 is a good example that includes two discrete tuners. "Hybrid" cards usually have a single tuner that can be either digital or analog, but not both at the same time. Also - make sure the TV card has a video input for your cable/sat box!