I built mine a couple of years back from some spare parts, works very nicely (obviously it's way overpowered for what it does):
-- 2.4GHz P4, 1.5GB RAM, GB LAN, onboard video.
-- Ubuntu 8.04 LTS installed on small IDE drive.
-- 6x drives (4x 320GBs and 2x 750GBs) as software RAID5 with LVM on top and ext3 on top of that, which allows RAID5 with different-sized drives and no lost space.
-- Azureus for bittorrent.
-- Fuppes for serving video and music to Xbox 360 via upnp.
-- Samba, VNC, SSH, etc.
I used to have some tuner cards and MythTV in it a while back, but I've since moved on to Virgin and a V+ box, so they've gone. I wasn't wholly convinved by MythTV tbh - the main thing I really miss from it is having access to loads of disk space, and programming recordings online.
Hardware-wise, I used to have a 333MHz PII running as a RAID5 fileserver with bittorrent, and that worked just fine.