I am currently reconfiguring my multimedia network machines to use Kubuntu 12.04.1 64bit, with Myth TV, and Samba shares to the server's external media drives, plus a few other things (OpenRemote & FreePBX, etc). I'd recommend Kubuntu (with or without MythTV).
For ease of use, consider using Cairo-dock,... moving the panel to the top of the screen and auto-hiding it, setting up easy to use icons in Cairo-dock, locking the configs for the desktop (otherwise count on fixing it, if your girlfriend is anything like my wife).
Also, I'd recommend;
* dumping the native video/audio players for VLC,
* installing the necessary codecs for media,
* making VLC default to full screen,
* tweak it's onscreen UI,
* setting VLC to auto-close on finishing playback,
* set it to suspend the screen saver, and
* setting the screen saver to cause the machine to suspend to RAM after a reasonable period...
Plus, you want to limit Nepomuk and Stringi to limit IO. For a nice multimedia remote, consider a Rii Compact keyboard with programmable IR remote.