Where are you located? If you're stateside, it's unlikely you'll find a digital PCMCIA TV Tuner card. For the record, I had one many years ago. It worked well enough on my basic Intel graphics, but again, that was an analog signal.
Six one way/half dozen the other. You could go either way (USB or PCMCIA). I'm on the other side of the pond, so I'm not familiar with TV signals over there. Just make sure the one you do decide on is compatible with your TV signal.
P.S. Maybe head over to http://www.thegreenbutton.tv/ for more information. They're definitely more Media Center specific and I know some of the folks over there are ...well... over there!