Broomster has a point.

I met a guy who purchases the apple mini, installs ubuntu and then
mythtv and then resells them as upgraded dvrs. Pretty cool.

If you wanted something very small - the Cappuccino pcs are really
small. I personally like the Shuttles. I have had some trouble recently
with my own computer but I have built many many of these in the past
with zero problems. Install 2 1TB drives, 2 video cards, quad cpu, 4 to
8gb ram and you could put it under the cabinet and it would be a awesome
DVR machine.