I'm by far no guru, but from my experiences capturing video on my PC (Celeron 433, 192Meg memory, ATA-66 40 gig drive, using an ATI All-in-wonder 128 PRO (software encoding) and a Hauppauge PCI PVR (hardware encoding) I had unsatisfactory results. I turned to buying a ReplayTV 3060 (actually, two of them) and pulling the 6MB/sec streams from the hard drive.)
My guesses of why the PCI cards failed are some of what was mentioned above. In addition, Windows is not a real-time operating system, so expect dropped frames, sound issues from time to time. There are just too many other events occuring in Windoze to guarantee a good recording.
Also, even though the Haugpauge PVR supports bit streams upto 12Mb/sec, my Celeron 433 & mother board began acting up with anything above 4Mb/sec. And even though the encoding is hardware, my CPU usage began was pegged at a 6Mb/Sec capture. Disk I/O was not an issue and I'm not sure why my processor usage jumped so high.
So, I wonder if buying any video capture for a PC running Windoze, let alone a HDTV will provide quality recordings. Perhaps under Linux with a customized kernel.
Well, that's my experience.