Macs are great for music.
Best stick with OS 9 for the moment though.
Logic Audio is great, and you can get a software sampler that integrates beautifully with it. It's called the EXS (I think).
For USB Midi, you could get the MT4 which is a low-cost 2in 4out MIDI interface that has VERY low latency when used with logic. It's quite a looker as well.
For recording Audio you could get a PCMCIA audio card. I think "digigram" - something do one. Do a search on google for "PCMCIA audio card".
Or you could get a USB box to get audio in.....don't use the built in sound ports - they simply aren't good enough.
Check
www.emagic.de
Good luck