I see what you are doing, and very soon I am about to start along the lines of the same thing, minus the fact that I need to raise up some money first
Anyhow, AnandTech did an article on this very same subject, and it's a verrrry interesting read.
Building a Linux PVR Part I
Building a Linux PVR Part II
Honestly I went parts shopping on Newegg, found a couple good tuners, chips, etc. AnandTech said Hauppage, so I'm probably going along the same route.