If all you are using it for is file serving MP3s to one person then 128Mb is fine, it will be quite happy, my Win2k server running my web site, database and serving MP3s to me had only 128Mb in it until recently and performed without any problems (it now has 512Mb though, as even ECC memory is cheap now).

If you are running apps on it interactively then 128Mb won't be enough though.