I found this link quite helpful. It's a rundown of what's available if you want to use your HTPC as a PVR and don't want to use the software that's supplied with the tuner card.
only OTA signals are going digital. If you pay for cable you dont need a converter box or anything.
But, i've got an HVR-1600 in my HTPC. It was kind of a b-word to set up if you don't have windows MCE