Having never experienced the VIA issues, I can't say for certain but I think it would be general instability, random lockups etc.

You could always just try it. It can be a fair bit of work getting a MythTV machine running especially if you're trying to add it an existing box due to all the dependencies. Have a look at Jarod Wilson's guide if you are willing to set up a Fedora machine from scratch - it's very good and has a lot of PVR350 info.

You could try and just get the PVR350 going with out Myth (you can jast cat /dev/videoxx or whatever to a file and then play it back with mplayer since it outputs MPEG2)

Personally I use the cheapest nForce1 board I could get (an Asus A7N266-VM and it's been rock solid for at least 18 months no PVRx50 though). You should be able to re-use everything - not sure on the T900.