all u need is a midi-to-usb interface and you're good to go...literally. u can run all kinds of apps...some only in 9.x tho...
(i use 9.2.2 for this reason)
i use reason, logic, peak, t-racks, all on a G3 powerbook that's (sadly) slower than the current ibooks...just get lots of ram.
figure out exactly what you want to do...
check out reason, logic...there's lots more.
have fun!