Last I heard, the state of Linux drivers for hardware video decoding did not make it worth it to Adobe to support it. But that news is about 1.5 years old, so maybe it changed in the meantime.
Edit: Flash 11 youtube 360p is using 14% of my CPU. mplayer 480p uses 4%. shrug
Depends on the video, the CPU, the GPU, and how the video is displayed in Flash, but yes, it should work better, more often (though I'm not sure the 64-bitness is that relevant compared with the other stuff).
It is. The nspluginwrapper used to run the 32-bit Flash in a 64-bit Firefox (dunno if it also applies to other browsers) really slowed things down. I got much better performance running the 64-bit plugin, all other things -- including the Flash version number -- being equal.
Depends on your GPU and drivers. I don't remember the specifics, but I believe only (most?) the closed source drivers can support hardware acceleration.
