Also, Logic Express 8 is FAR easier to use than previous versions. Certainly worth checking out. Almost everything you know about GarageBand is the same in Logic, but you get more options, like support for external MIDI instruments, tons of additional plug-ins. Its a lot of fun.
As for GarageBand. You can always install a free Audio Unit plug-in that gives you MIDI output. You'd use a software instrument track to send out MIDI, and an real instrument track to record the output of the Proteus.
Its fairly painless, but ironically, easier and more intuitive in Logic Express.
MidiO 1.2 is a universal binary (works on PPC and Intel Macs)
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