If you want to transfer files to your computer, Replay is the way to go. It will not record any digital stream directly, but if your satelite company doesn't do locals, a dish PVR won't record your local channels anyway.
I personally record everything at low, and it will fit 4 hrs on a DVD. Low looks fine to me on a 32" screen. If you have a bigger screen you would record on high. Low 1gig/hr, med 2 gig/hr, high 3gig/hr. Replay can output a digital signal, but most people are happier with an s-video connection since the TV's have better line doubler circuitry.
If you main reason to by a pvr is to record for longterm storage, you might consider a stand alone DVD recorder.