Just to let you know, mythic.tv has a few tuner cards and also the just mentioned HDHomerun box (it's a little box that sits anywhere on your network and has two tuners - I believe both for cable or over the air - then it just streams the data over the network to your computer).

They also have a listing of the components they use in the Dragon Myth system - aka the Knoppmyth Reference Platform. They will build one for you or just give you the idea for good stuff to put into a box (they do sell some of the internals by themselves). I based my system off their listing and it works well.