I wouldn't install Nvidia's driver versions 100.14.19 or 100.14.23 if you want to watch video on MythTV. There is a bug which will corrupt the video and you will get a bunch of boxes on the screen. The only thing you can do is use 100.14.11 (you will need a patch
if you use 2.6.23) or try the latest beta, 169.04.
The first thing I would do is to verify that you can watch video using your Hauppage WinTv Live Go-Plus. I am not familiar with this card, but I do use a pvr-350, so hopefully I can help out. The first thing I would do is to use mplayer or vlc to watch the video stream. To do this, you need to run "mplayer /dev/video0" from the command line. This should produce a watchable picture. If this doesn't work, then you probably don't have your card setup properly. If it does, then you don't have MythTV setup properly.
Also, according to this wiki
you need to use special settings for this card. What is in /etc/modprobe.conf? Is there a line to load the bttv module? If so you just need to add "options bttv radio=1 card=26 tuner=39 gbuffers=10", reboot and it should work. If not, then you are going to need to add lines to load the bttv module. It would probably be something like
alias char-major-81 videodev
alias char-major-81-0 bttv
options bttv radio=1 card=26 tuner=39 gbuffers=10
Also, be sure to read Jared Wilson's MythTV how to
for more help.