I have a HTPC setup using mythtv, it is actually second one. I upgraded it a few years back, it ran MythTV before that. I've been using MythTV since around version 0.17 and it has come a long way since then.
My video archive is stored on a seperate computer than my HTPC, it has a 1.5TB RAID 5 array (which is slated in June to be replaced by a 4TB RAID 5 array). And I just mount the drive over the network on my HTPC.
My HTPC hardware currently is:
2.4 Pentium 4 Northwood Core
512MB Dual Channel DDR400 memory
80GB SATA1 HD
Hauppage PVR-150 TV Tuner card
128MB nVidia GeForce FX 5700 Ultra
Antec Sonata 1 case
It used to have a GF4 MX 440 video card, I only put in the FX card because I needed a DVI port to connect it to my HDTV when I got it. The machine is pretty much dead silent, I have it in my living room and you can't tell it is on from the noise alone.
The hauppage cards work the best under Linux. However one thing that I highly recommend is making sure your capture card has a hardware MPEG encoder onboard. It will seriously reduce CPU overhead and allows the audio to be multiplexed in with the video stream. They [haupapge] has some pretty nice HD capable TV tuners that will do ATSC and QAM, they are reasonably priced as well.
I am planning this year to upgrade my HTPC again because my setup cannot play H.264 720p video. Since Linux does not have (but it is close to arriving) direct rendering support where a lot of the video decoding can be offloaded to the CPU.
I use a remote control for my HTPC that uses LIRC. I blogged about it when I build the receiver module: http://www.oracleguy.ws/entry.php?entryid=90
The stuff at the bottom of the post was for trying to get the IR receiver to work on a laptop (it works fine on my HTPC directly).