Right now I use a P4 2.4 PC running Diginet, HAL2000 and to watch an occasional DVD in my bedroom. I previously had Diginet running on a P4 2.8HT and was able to also use the same PC as my primary TV DVR and bedroom DVD player.
Diginet doesn't require it's own physical HD for recording, but does require at least one dedicated HD partition. I use an old 80GB HD for this, which gives me about 1-2 weeks of recording 5 cameras at 640x480, 30 FPS.