First, if you haven't done it already, go buy yourself a copy of Crossover 6.0, it will be necessary to make this work, plus we need to support the Wine project! After that, download the latest Wine from the git tree. I tested on version 0.9.31 from git.
Once you have Crossover installed, run it and install Internet Explorer 6. Choose the "Customize your browser" install option and make sure to get the offline browsing pack, extra fonts, and Windows Media Player.
After you complete installing IE6, go download a copy of Windows Media Player 9. Install it to the same Crossover bottle you just installed IE6 into, using the "Install unsupported applications" option. Installation will claim to fail at the end, but don't worry, it will still work.
Now you will need to download the Windows Media Player 9 codecs package and install it.
Install the Napster client software now, ignore the message about Windows 98 not being supported, and go through with the install. I also recommend unchecking the Napster system tray icon, because it is stupid...
Now for my super slick secret trick, I like to make Wine and Crossover share the "bottle", don't worry, Crossover won't get the cooties. Create a symbolic link to your bottle like so:
ln -s /opt/cxoffice/support/managed_win98 ~/.wine
ln -s ~/.cxoffice/win98 ~/.wine
Then run Napster on Wine like so:
wine "C:\Program Files\Napster\napster.exe"
Tada! Now, if you could only buy the music... a topic for another day.