Ahh... good old mythbuntu
It is nice to use when you just want a media PC and you want it up and running quickly. Takes care of a lot for you and as long as you're sensible, in my experience most of the time it gets it right. Nowhere near as flexible as doing a custom install on something like Gentoo, but not many people need or want that degree of customisation these days.
The main thing to check is that your tuner card is compatible and works - have a look here
to check if it's listed as tested. If it's not a V4L compatible card then you're pretty much stuffed from the word go, so be sure to double check this!
Less important these days but it's also worth doing a general check over of the system and making sure the sound's supported too - occasionally with a strange or new card you can run into issues here. But that goes with any linux box
Only thing that errs me on the side of caution is that you say you're installing other software on there as well - I've had strange issues sometimes when trying to install other things on these sorts of distros, and while they're generally not too hard to resolve I'd still personally be tempted to grab a normal Ubuntu install and install MythTV on there. That's really just me though, I was using Gentoo long after most others ditched it as not worth the hassle - I wouldn't say you're doing anything wrong by going down the Mythbuntu route.
One big advantage though is that it installs pretty quickly and the set up process is menu based and painless most of the time. (Make sure you select to install the back end and the front end though, you need both unless you're planning to do some clever distributed stuff and have the server somewhere else!) Obviously being based on Ubuntu which is one of the best out there for plug and play stuff these days helps a lot in the hardware respect. So if for any reason you can't get it working, you really won't have wasted much time trying it out
There's others out there as well but I'd say mythbuntu is probably the easiest one to take a shot at and there's a pretty good chance it'll just work - always a plus!
Hope you get on with it well!