Compaq DC7700 Small Form Factor
The HP Compaq DC7700, 7800, 7900 Small Form Factor (SFF) is an excellent PC for running MythTV. HP's page for the DC7X00 
It's compact size, minimum Core2 Duo CPU, and super-quiet operation make it an all-around great host for an HDTV card at excellent prices. A 200GB+-size drive is the bare minimum necessary if you want to store a variety of shows in HDTV.
A couple of notes regarding this PC.
The DC7700 and follow-on models (7800 and 7900) have been around for years and the available slots specifications have changed. Pay careful attention to the specifications as they vary. For example there are some models with both a PCI express 1x and PCI express 16x slot. However, the slots are too close to put a PCI-express graphics card AND a PCI-express TV tuner. Sometimes you can find them with better graphics than just an Intel GMA.
The Intel GMA on many models is sufficient for rendering HD with a number of options enabled in the xorg.conf file. It also helps to minimize latency on the PCI bus.
Example 1: setpci -v -s "*:*.*" latency_timer=b0 #this reduces latency of all devices on the bus. Don't just add this to your system start.
Example 2: setpci -v -s 04:01.0 latency_timer=ff #The better way is to update the device you want to optimize, like the audio and video device buses.
The available PCI, PCI Express slots on the motherboard are half-height. That is, you cannot use an HDTV card, or PCI Express graphics card with a full-height bracket. There are a couple of ways to resolve this issue.
1. Order a PCI or PCI Express riser card to use with your full-height cards. HP's link with part numbers: http://h18000.www1.hp.com/products/quickspecs/12376_na/12376_na.HTML As an extra precaution, make sure the kit includes the back plate when you order from your favorite reseller. The riser card gives you the ability to use a full-height card at the expense of covering the half-height slots. For example, you cannot use both a full-height tuner card and a PCI-e graphics card.
2. Use low-profile cards. Low-profile cards are very common. In fact, many graphics adapters include a low-profile plate in the box for just this reason. Hauppauge's newer cards can fit in the low-profile slots. Space is tight though. So, a PCI-express graphics card may not fit next to it. Be sure Hauppauge includes a low-profile case bracket before you buy. [May 2009] I have taken delivery of a Hauppauge Nova-T 500 which included a low profile case bracket.