Here is a low-profile PCI-E TV card which should capture your attention. The Ceton InfiniTV 4 card is similar to those usual TV cards which allows you to watch and record TV shows via your PC. But the amazing part about this one is that it can record 4 HD shows simultaneously!
You will need to add a single multi-stream CableCARD to make it work though. But once you do, just run the TV Setup via the Windows Media Center and you are good to go. The TV card retails for only $400.