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).
I used a Pentium 100 MHz and it would play just fine. Encoding them would take real-time (i.e. a 3 minute song would encode in 3 minutes), but playing back was fine. I even listened to music while doing other things. This was with Winamp.
My intel box may have been a 166Mhz machine. Either way this assumed you weren't trying to do anything else. The PPC mac was definitely 100mhz though - maybe Apple marketing wasn't a lying about perf/clock back in those days.
