When you create a new Software Instrument track in Logic 8, there is a check box called "multi-timbral" in the New Tracks window.

Check that box and put "8" in the Number field (so it will create 8 tracks).

Then assign Stylus RMX as your I/O on the first Instrument track and Logic will automatically configure it so that each of the 8 tracks in Logic correspond with the 8 multi parts in RMX.

You can then drag the MIDI data for each part in the Stylus RMX Multi to the corresponding Instrument track in Logic.

If you need the auxiliary routing you had in Logic 7, you can set it up the same way in Logic 8's Environment as it is detailed in the Logic 7 video on the Stylus RMX tutorials disc.