I built one just recently.
I had been using a Via Epia M10000 with a Leadtek WinTV running MythTV and Linux. But I wanted to try to get QAM w/ HD, generally a faster and more responsive system, and I wanted a better remote control, and better ability to move the TV shows over to my iPhone and it looked like I was going to need a fair bit more horsepower than the Via M10000 could give to move to the next level.
My goal with these things is cheap, and quiet... so I scrounged around the house, bought a super cheap CPU and bought an open box motherboard from Newegg and came up with my new one with is based on a 2.6GHz Celeron CPU, 1GB Crucial DDR2 memory and a MSI PM8M3-V motherboard. I used an old leftover AGP video card (some nVidia 5000-series, I think), and an old hard disk, the XP license from before I upgraded my main computer to Vista, a really old ugly beige case with an even older power supply and then a spare 250GB hard disk and DVD drive. Total money spent was $70 for the tuner, $25 for the motherboard, $30 for the RAM and $40 for the CPU, everything was spare parts. No keyboard or mouse... I just use the remote for navigating Windows. My TV is a 37" 720P LCD.
I splurged on the ATI HD650 tuner card and remote. For software, I looked at SageTV, Vista's Media Center and BeyondTV and settled on BeyondTV. I've had it running for several weeks now and it's working great. I like the compatibility with my iPhone - it generates H264 videos automatically, the video quality is good, I love the way BeyondTV handles commercials - it can remove them all by itself, the HD650 remote is way better than my old Leadtek and is very responsive. About the only thing that I don't have is the clear QAM, but I'll keep working at it and I should be able to get something.