I highly, highly, highly recommend Jarod's Guide. Bare metal to working, full-feature system with Fedora Core 6 in a couple of hours. You'll want to do a kitchen-sink install, play with the system, then go back and do it again; the second time will be faster, you'll be better at picking out what you want and need, and you'll be happier with the result. I also rebuild the systems once in a while, partly from a need to tinker.
But otherwise I have no experience of MythTV.
Very, very tempting, if I could only scrape together the money. Ah well...-- This is not a psychotic episode. It is a cleansing moment of clarity.
I'd rather buy something off the shelf the next time, given that they can tell me that it's quiet. Then again myth-tv probably has an easier HCL than windows mce, giving you more room to wiggle about.--
The revolution will not be televised.