I know from me this will sound way out there but, skip the mythTV and get either a
http://www.dlink.com/products/?sec=1&pid=387, put together a cheap machine and put Windows MCE on it or get an XBox with XMBC. Its considerably easier to setup, does everything a mythTV does and more.
I replaced my myth box with an XBox running XBMC. I dont regret the change for a second, not to mention the XBox outputs in HD and my myth box didn't. Keep in mind however, I dont use my XBMC to watch TV, just to watch movies, DVD's and play music from my fileserver, have a HD DVR from comcast for watching TV. If you plan to run your TV through it, go with the windows MCE. As great as linunx is, a myth box just isn't the best for what your trying to do.
If mythtv is really what you want to do, make sure you have products that are sure to have drivers, especially your TV Tuner. I would read throught he mythtv howto before getting any of the hardware. I hear Knoppmyth is good, though I ran mine on a slackware box which was a major pain in the ass to setup. I am sure it would be much easier on a gentoo or fedora box where you can emerge/yum the install.
For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill.