I played MP3's on a 486 DX4/100mhz. There was a setting in winamp I had to turn on (I think it was quality related) otherwise it would stutter and be generally unlistenable. Even with that setting turned on it pegged the CPU at about 100% the computer was unusable for anything else while the mp3 was playing. Trying to seek in the track would sometimes crash winamp.

My Pentium 166 on the other hand was much more capable I could multitask while playing Mp3's (I used to play mp3's and Quake at the same time).