I recently purchased a cheap nVidia 5200 and I can't get it out of "performance mode 0". I even tried all the registry switches listed here.
I put it in an old 1ghz P3, hoping that I could playback youtube resolution videos but it can't do it. I seem to remember much better performance with an older nVidia card I had in there at one time.
I really couldn't tell much difference between between playback when using the "nvidia", "nouveau" or "nv" drivers.
Either I got a bum card, the card sux or my old 1ghz P3 is just too slow (which I don't think it is).