A card with the 550 Pro in it gets my vote. It does make a difference in picture quality, and you're better off with hardware mpeg-2 decoding as well, especially if you have a lower-end processor.
As for what a TV Tuner can do, essentially all they do is capture an analog NTSC encoded signal and encode it into an mpeg-2 stream. They also encode the audio component into a transport stream, not sure if it is mpeg-3 or what. Some have multiple inputs so you can encode from more than once source (but not at the same time unless you have more than one physical tuner in the card, or multiple cards). There are also some cards available now that decode QAM compression and allow you to receive digital cable channels. Some also have HD capability, and some have both QAM and HD.
Everything else you can do comes from the software that handles the mpeg streams. The software is responsible for telling the card what channel to tune, and for doing something with the video and audio. That means either displaying it onscreen or writing it to disk. I have tried PowerCinema, Beyond TV, and GBPVR. PowerCinema sucks, period, as does almost anything from Cyberlink. I couldn't get GBPVR to work. I got a retail copy of Beyond TV 4.0 for Christmas and I am 100% satisfied. Well, maybe 97%. I wish it handled volume a little differently. But other than that it is one hell of a slick program. They have a version that comes bundled with the Firefly remote, and that has also proved to be 100% reliable and very responsive, unlike the ATI Remote Wonder that I tried previously.
So my recommendation for ultimate PC TV goodness would be: widescreen monitor, surround speakers, QAM/HD capable tuner, and Beyond TV. The budget package would lean toward the lower end Hauppage or ATI cards, and would still include Beyond TV, because without a solid software front end the card is just taking up a slot.