It depends on what powerbook you have, the 12 inch doesnt have a slot for a sound card, but you could always get a usb or firwire interface if need be, also what speed g4, how much ram and how fast is your hd? At first I would'nt worry to much about a sound card untill you've spent some time producing, and really you will need to be making songs with alot of tracks and effects before you will have to worry about upgrading, and you won't be making songs that sound good with a ton of tracks and effects untill youv'e learned atleast the basics of eqing and mixing. A fast (7200 ) HD is gonna help alot, so will ram.

As far as software goes, I would go for logic right from the start if I was you, and then start working on getting plugins. There are a million different options out there really, this is just one.

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