I'd been wanting to build a MythTV box for a while and finally did it last year. I've got a SFF case, a uATX motherboard and an old cast-off video card from my gaming rig. It runs Ubuntu off a SSD and has local storage so that wife/kids can still use it if the network goes down for whatever reason.
It was relatively easy to get up (mostly) and performs very well as a PVR. Getting myth to work with old downloaded videos is a little bit more difficult, PlexTV does this much better. I've also got Steam for Linux installed, with the intention that the kids can play some games locally on the TV or play things from my PC using Steam's in-home streaming. This doesn't work anywhere near as well as I'd like it (but it's running over WiFi, so YMMV if your lounge room is cabled in).
You can fairly easily setup a menu item in Myth to launch NF in a browser window (I haven't done this yet).
If / when I move house, will I do anything differently? Not sure. Having a separate backend makes more sense to me in a geek / network design kind of way, but I think in my case there are definite drawbacks (ie low WAF, cranky children) if I'm fiddling with the server and it goes down.
I'd like to get Steam working properly in the lounge so 1) kids can be supervised while playing games; 2) social gaming; 3) no crumbs in my keyboard and this makes me lean to doing a windows-based media centre next time, although that may complicate myth setup.
There's no reason you can't run Plex and Myth on the same server - use plex for NF and watching movies and myth as the VCR. This also gives you the mobile power of Plex (syncing 40 episodes of Peppa Pig to your mobile / tablet for the kid in the car, anyone?)