Better Quality from HD tuner card?

[joel]

I have a couple of analog tuner cards in my HTPC. A Hauppauge (dual) and an ATI card. Recently got a cable box/dvr. I am running svideo from the HTPC to TV. Composite video from cable dvr to TV. Composite from VCR to TV. A direct coax feed to TV tuner. The HTPC, VCR and TV tuners all have good pictures until I compare them to the cable box picture which is far superior. (Actually the cable box feeds into the VCR AUX port and then to the TV, so the VCR and cable box are going to the TV via the same composite cable)
I can only conclude that the cable box/dvr has a superior tuner (may have to do with its digital reception?).

My question is, if I buy an HD tuner for my HTPC will I get a comparable picture to what I see from the cable/dvr?

Any suggestions for a reasonably priced HD tuner?

Thanks,
Joel