I'm heavily invested into MythTV, and I'm always looking at ways to make the frontends smaller and cheaper. I'm always looking at ways to integrate MythTV better with my needs.
Here's what I found. If Live TV and recording playback is your primary focus, stick with MythTV for both your backend and frontends.
XBMC is slick, but MythTV does it really well also, plus it handles your live TV and recordings. Then you're not fumbling with multiple interfaces.
Comcast in Chicago allows all channels to be be played and recorded, except for premium channels (HBO) with cable card. I can't speak for other areas of the country, but surprisingly, Comcast is the most friendly with the HDHomerun Prime.
For a backend, use your most powerful available computer. Especially if you're streaming to multiple frontends. My MythTV backend operates as a frontend, backend, but also a Subsonic music server so I can stream my music library to my iPhone/iPad and other devices/computers. I'm doing that with a Core 2 Duo with 4 GB of RAM. I have one other frontend that's constantly on. Seems like the computer handles everything fine, but would I suggest going smaller? Probably not.
MythTV is not the most friendly program to set up. If you've never done it, prepare to pound your head on the desk for awhile. There's a lot that's not very intuitive. The MythTV developers don't have a marketing department to go through their processes to make them more user friendly. So you end up with incoherent ramblings of brilliant people.
I don't suggest prepackaged MythTV distros like Mythbuntu. I also don't recommend the route I eventually took, which was Arch Linux with MythTV built into it. Start with a simple Linux distro like Mint XFCE and add MythTV from the repository and go from there. You'll have a much easier time getting it running that way.