To be clear, the digital cable box allow you to view encrypted digital channels (Ex. ESPNHD, HBO HD, NFL Network, etc). Channels you cannot view via regular analog cable (cable direct to your TV or tuner).
If you want to be able to view these channels in an SD resolution, then all you need is an IR blaster, so BTV can change the channels on your cable box. The PVR-150MCE card will work just fine. A bit more complexity here is changing channels via serial cable. This is more reliable and faster since it's a hardwired solution. The problem is that not all boxes have a serial input, and some that do just can't make use of it. I have an old Motorola cable box (non-HD) and it has a serial port that works on it. You can find a null modem cable (serial cable) for cheap. I got mine at Fry's for $5.
If you want to view HD shows at HD resolutions, then you'll need an HDHomeRun as mentioned. Setup is not straight forward though. With it you can view and record any unencrypted digital cable shows (no cable box needed). Most, if not all, cable companies leave their local cable shows unencrypted, so you can view and record shows from NBC, FOX, ABC, and CBS in HD.
HDHomeRun instructions:
HDHomeRun QAM + BeyondTV Instructions
http://projecthtpc.wordpress.com/200...-tv-hdhomerun/