Linux is pretty damn easy. It only gets harder when your needs become more complex. Then again, Linux being "hard" is subjective.
Linux on the HTPC is pretty good for basic uses but it depends on what you're doing. If you were looking to build something just to keep all of your movies and music on it, you'd probably find it easier to just use Windows Media Center in Vista or 7.
Now if you were talking about a rig to store and stream all of your media to various PC's around the house, a MythTV backend can be awesome.
As for hardware, it's been a while since I took a look at what MythTV was compatible with. I've heard from those before me that MythTV is a bitch to setup though. It could've changed in the last year or so, but I doubt it. Eitherway, there are plenty of guides and how-tos online to help you out along the way.