First think about what you want to do. Will you be building a backend that is in another room? Than you might want to fo for a "Big PC" with lots of noisy vans and an ugly case and your frontend might be a slick barebone or even an XBOX.

If you want to have an "All-in-One" system, you might want to go for a neat barebone that is silent and a TV card than can do hardware encoding/decoding.

Then check the hardware. And before you check the hardware, make sure to check the hardware. And again. I cant stress this enough, make sure that the hardware you select will work both under Linux and under MythTV.

Google for hardware and MythTV, go to your hardware store and ask if that socket fits in that motherboard, if there are enough PCI/AGP slots, if it isnt a mini PCI slot. Then with that complete list google again to see if people build something like that. Great resources are Build Your Own PVR and PVR Hardware database. In the links session are more usefull links.