It depends on the quality you want. For SD, most any video capture card that supports composite should work. Even a tuner card will work if it supports analog tuning (some don't) and if you tune it to channel 3 and use the RF output, although this is worst-cast.
Basically, the video output is just standard video. That said, it is possible for MacroVision signals to be encoded, and many digital recorders and input cards may block the signal if they detect it.
I am revisiting this issue because I still have the recording saved on my Comcast DVR and I need to transfer it to my PC. My plan was to purchase a video capture device for my PC, play back the recorded episode from my Comcast DVR, (through the video capture device) and save it to disk. But when I started researching, I got very confused about whether any of these devices will recognize the playback signals coming from the Comcast DVR. Does anyone know if this will work? If my television can play back content from the DVR then is there a PC device that can do the same? OR... Can I play back the DVR content to my television and connect an output from the TV to a recording device?
Any suggestions on what brand, type, technology etc will work?