Hi,
The machine sounds up to the job
It should be possible to install KnoppMyth and use it without a TV card, but I've never tried it. Most of the functionality and purpose of MythTV relies on having a tuner card. DVD playback is currently handled via the MythTV frontend running another application (xine) anyway and the MythTV backend (which handles tuner cards, recordings, programme guide etc) isn't involved, it might be an idea to install Ubuntu then install xine (which will play DVDs as well as various file formats and streamed Internet TV etc.)
The same X modeline alteration for 405-line will work on any Linux distro. (A bold statement, there; but all Linux distros providing a full GUI use The X Window System, either the XFree86 or X.Org incarnations; both use the same nVidia binary driver, and so ought to work. I've run 405-line systems with both XFree86 and X.Org)
KnoppMyth itself is 'in development', I find it pretty easy to install; but it's not as easy to install as Ubuntu is (provided it likes your hardware, you've not got a broken router, etc, etc <sigh>)
MythTV itself is also very much 'development software' - the current version is 0.19
It might prove easier to give Ubuntu a try, get DVD playback working on it with xine or VLC, then tweak it for 405-line. Maybe. I should try it myself
If you're intending to "go the whole way" with MythTV eventually (I'd recommend the Hauppauge Nova-T PCI DVB-T "FreeView" card myself, I have two of them in one machine) then KnoppMyth is arguably the way to go.
I'll dig out the schematic for the RGB/sync combiner tomorrow, turn it into a .pdf then upload it somewhere.
Regards, Kat