Приветствую, Theunis
I think it is your card that is the problem
yes, I think so GeForce 6150SE nForce doesn't support vdpau
http://en.wikipedia.org/wiki/VDPAU shows that is should support up to VP1, however if you look at mythtv's guide: http://www.mythtv.org/wiki/VDPAU it starts only at series 8 and newer.
It seems that your card can then only support xvmc for mpeg2 codec only. :(
Buy a a new 8400 GS (old brand model name but new architecture G98 chip) as indicated here http://en.wikipedia.org/wiki/Nvidia_PureVideo
It is cheap and runs at 40'C, fast and silent (mine has no fan on it) and supports up to VP3.
I'm using PCI 8400 card from Sparkle and I would recommend to buy more powerful card - for advanced deinterlacing as temporal_spatial. Unfortunately my 8400 card could manage well with temporal_spatial for 1080i video
Goga