my roommate built a mythTV box about a year ago, it works great.
Hardware wise he's got a P4 (unknown speed) and probably 512mb RAM, two 250 gb discs in a 500gb raid array.
We had a 6800 in it until that card died, now it's I think an old Mx440.
I don't know the make of the tuner, though it you want it I could ask him.
It's a great system, movies, TV shows, SNES games, N64 games (until the 6800 died, the 440 can't handle them).
Just don't use an Ati card, they really don't have much in the way of drivers for linux systems, and what they do have doesn't work all that well. We tried a 9800 after the 6800 gave up the ghost and it got us nothing but headaches.