I just played the tron legacy trailer in 1080p from
(from youtube) with smplayer - i let it to cache all the file, go into /tmp folder and copy the file from there (has a random name, but you see the icon), it can be played with any movie player.
M3N78-VM mobo (using the onboard 8200), Athlon II 250 @3.0GHz processor, 2 GB DDR2 @800 MHz RAM, Debian Squeeze 32-bit, smplayer/mplayer from the debian multimedia repo, nvidia drivers 256.44 from the nvidia site.
The playback is perfectly smooth, no hiccups or anything, mplayer eats 3-5% CPU, smplayer 3-5%, the CPUs stay at 800 MHz.
Id say the problem problem related to your ffmpeg/mplayer.
I have adaptive clocking anabled, but only 1 performance level exists and no options to change anything related to it. I have a laptop with 135M card and that has 3 levels and i have the option to choose adaptive clocking or maximum performance. But this card dont seem to have performance levels.
Also the memory clock is shown as 400, i remember sometimes back (dont know exactly what driver version) seeing 800 at its speed.