If you are doing a studio in any professional way, you will want an external multi-channel interface of some description. Conventional wisdom is that the minijack audio in and out of a computer is not good enough quality. The Optical connection of course doesn't suffer from that limitation, but, how would you get microphones plugged into an optical input? - you'll need outboard equipment regardless. Look at Edirol, Mark of the Unicorn (MOTU), Presonus, M-Audio, Digidesign. Start reading magazines to bring your self up to date on the technology available - Sound on Sound, Recording, Electronic Musician, Keyboard, Computer Music, Future Music
Go for the 17". You need all the screen acreage you can get for showing the controls of software mixers, eq's soft synths, samplers, etc. Get the Superdrive (DVD-R) model; you *definitely* want to be able to back up projects larger than 700 Mb onto optical media.
Get a Firewire 800 external drive like a LaCie Big Disk Extreme. You will want that room for storing project files and backups. You want to use FW800 for storage both for the improved speed, and to keep the data stream off of the FW400 buss, which you will most likely want for the audio interface.
Dig up some backissues of Recording Magazine and look for Bill Stunt's articles on creating a mobile studio with a Powerbook G4.
Thanks
Trevor
CanadaRAM.com