in the early Pluto days (the ancestor of LMCE) I started my testing with a VIA EPIA M 10000.
I also tried Mythtv but result were not brilliant, probably because at that time MythTv integration was very rough and unstable.
Then I moved my hybrid on a dedicated box, so my VIA box became (an still is) a regular diskless MD.
I had to tamper a bit to have decent video performances but in the end the result was good using Openchrome drivers.
With LMCE things are a bit different. Openchrome drivers (shipped with Kubuntu) are giving some problems. In my case they are crashing MD when watching divx files, so I had to move to Unichrome Pro drivers (you can easily find in this forum some useful pointers on this matter).
Note that with Unichrome Pro you cannot have UI2, as this interface is only supported by Unichrome Pro II drivers that have been licensed from VIA to Fiire.
As Eloy said, if your family is used to rely on MythTv I guess that killing you working system and reinstalling as LMCE would be a risky option.
I never used Mythdora, but from what I've read i think that in terms of "horse power" requirements LMCE tends to be more demanding that your actual system, and this may negatively impact on performances.
I don't mean to be discouraging, but I think you better find a way to introduce a new "electronic friend" to your wife
Just my 0.02