I found my A1200/060/50/AGA fine for MP3s with Amplifier although obviously system responsiveness is in proportion to the settings of the MP3 decoder, i.e. midway sampling was fine, high sampling made the system a bit sluggish.
As for mpgs, as others have said I found this okay but not brilliant on AGA, I am sure it would be fine with a gfx card. However support for the burgeoning number of video formats apart from mpg would be another issue. I found RiVA and MooVid good, and never had any luck with Frogger, occasionally it would actually play something.
Under AGA certain graphical operations can be massively accelerated by using a planar-to-chunky patch like BlazeWCP, the excellent FBlit and for some things FScreen although some programs will not open a screen properly with FScreen which was a shame as it made browsing especially much faster (virtually no "AGA ripple").
For FPU functions use a patcher like Cyberpatcher or Oxypatcher etc. I found Cinema4D multiples of times faster with Cyberpatcher on than off!
