AVS Forum rocks for these topics. I don't know anything about DirectTV. Generally speaking there are two functions involved in a TV tuner card: tuning and signal conversion. A tuner card makes the most sense when you just want to drop a raw signal into the card and use it to tune and capture a channel out of all that RF spectrum. This is becoming a rarer scenario with the growth of digital-only broadcasting and satellite/FIOS.
So the first question is: what do you want to tune with, and where do you want to control tuning from. I think with satellite you have to tune at their STB, but I don't really know. If that's true, and you are willing to tune the sat. box then watch the output on your computer, _and_ the sat. box has analog or s-video out, you can use any capture board or tuner card (ignoring the tuner part).
The second question is what kind of data do you have access to from the satellite feed? Analog NTSC out is the worst quality, but provides the most options for getting it into the machine (composite or s-video). Anything other than analog raises a host of questions. Can you get unencrypted QAM from it, via firewire or some other interface? Then you can use a digital QAM tuner like HD Homerun or MDP-130.
I would so some research on the Direct-TV equipment on the web and figure out what sorts of outputs they support (if any), and take it from there.