get yourself one or two of something like one of
these, set up a myth back-end somewhere. the biggest problem with myth though is it's front end UI sucks, and the developers are actively hostile to coming up with a standard that other people could make alternatives for. but if you can handle not using a mouse and the TV program taking over the computer while you are using it (or only watching recorded shows) then this can work well.
alternatively you can use WMC, which has sort of the opposite issues, it has a really slick UI, but it's functionality is lacking (for instance you cannot watch TV from anywhere other than the computer with the tuner). but for a single computer solution it's the way to go for now.
also keep in mind nothing digital will work with any* tuner cards right now, so everything will be low def.
*only unencrypted channels can be tuned, but everything except QVC and the weather channel are encrypted, there are some tuner cards capable of accepting a cablecard, but they cannot work outside of a fully proprietary environment, this is not merely saying it won't work in linux, you need special bios special version of windows and special drivers to get them to work properly
