What's the minimum requirements for Xubuntu? Any way that you look
at it, a machine with a 300 MHz processor and 256Mb of RAM is not going
to be mixing much music.

On a machine with a 300MHz processor, I'm guessing that the best that
can be hoped for is an XFCE distro (Xubuntu). More than likely a
Fluxbox distro (Feather Linux or Damn Small Linux) running lighter
applications will be required for a 300MHz machine. By lighter
applications I mean stripping out Gimp and OpenOffice in favor of
something a bit lighter like Abiword and gThumb.