Compaq DC7700 Small Form Factor
The HP Compaq DC7700 Small Form Factor (SFF) is an excellent PC for running MythTV.
It's compact size, Core2 Duo CPU, and super-quiet operation make it an all-around great host for an HDTV card. A 200GB+-size drive is the bare minimum necessary if you want to store a variety of shows in HDTV.
A couple of minor notes regarding this PC.
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.
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. It's unclear if Hauppauge includes a low-profile case bracket. [May 2009] I have taken delivery of a Hauppauge Nova-T 500 which included a low profile case bracket.