If you are not planning on building a PVR/DVR/Media Centre, ie are not planning to record Freeview TV, then MythTV is possibly overkill.
IT also depends how integrated you want it to be, by that I mean would you be happy with a small PC or laptop that sits under your TV and you walk up to it to play/pause/stop video that is just connected from the VGA out of the laptop or DVI of a desktop to VGA/SCART/HDMI of TV. Or do you want a shiny box that looks similar to other Home Entertainment equipment that you control from the comfort of your sofa via remote control?
The former is a doddle to setup, the later would require a lot more effort.
My dad just uses a VGA to VGA cable from his laptop to the TV.
I've got a DVI to HDMI converter, HDMI to HDMI lead from PC in shiny silver box to TV. The PC has two Freeview tuner cards and can record/play 8 channels at once operated by a Soundtouch iMON remote.
The MythTV wiki is a good place to start. But really, if you just want to play your videos you already have, a standard PC/laptop with Xubuntu and VLC + Mplayer installed would be fine.
I'm starting to think I should build MythTV boxes for a living and jack in the day job
Wyliecyote might be up for limited liability partnership