ATI doesn't have video acceleration under linux. However, ATI seems to be working on a new driver which would bring support for VA-API. (As far as I know).
On Linux, you can use "ffmpeg-mt" (if you got multi-core cpu, it'll decode the movie with X core) or just give extra parameters for mplayer.
Like: mplayer -fs -really-quiet -vo xv -ao oss -vfm ffmpeg -lavdopts fast=1:skiploopfilter=all $file
You can give these parameters in smplayer, options, advanced.
Just type the following there:
-fs -really-quiet -vo xv -ao oss -vfm ffmpeg -lavdopts fast=1:skiploopfilter=all