Okay, here goes...
My 4-year-old G3/400 Powerbook (Pismo),--now 500, due to last weekend's upgrade--with 1G RAM handles OSX.3.9 and Digital Performer 4.12 quite nicely. I've gotten at least 16 tracks at once with no issues, using a MOTU 896 and 10G external HD. Most often, I take it down to my drum booth, record 8 tracks, take it back up, and dump it to my G4, where I continue to add to it.
My Pbook is essentially the same as the second generation iBooks (the first ones being those cool clamshell ones, that came in green, blue, gray, and orange); my motherboard is nearly identical to the G3/500 iBook from 2002/03 (one of which my father-in-law has and works quite nicely on).
You can get a G3/800 iBook with a 14" screen for a pretty decent price; max it out on RAM, and there you go. Hell, you could probably go with a 600 Mhz, even, and still be very happy.
As with any laptop, screen real estate is at a premium, so a little later on you can add a 15" LCD monitor as an extension and go dual monitor (as I do, from time to time) on your iBook. It works.
For interface, I'd try to pick up a MOTU 828 (RME Hammerfall makes some great ones, but they rarely go cheap, even used). I realize there are cheaper 2-input solutions, but I fear that the more you use one of them, the more you'll wish you had a more robust interface.
Lots of variation in the price of iBooks; try smalldog (smalldoggy.com, I think), eBay, or a local place (in Denver, anyway) called themacoutlet.com. Joseph's prices are a little higher than what I've seen to be the general average, sometimes, but his service is excellent and he's easy to work with. His shop is a great way to kill a couple of hours on a Saturday afternoon, if you know what I mean.
Hell, if you wanted to get a great deal on a Pismo/400 (two Firewire ports, yum) there, I can tell you that the processor in it is first-rate...'cause it used to be mine. It'll probably go for about $375-400.
Another thought: craigslist.com in your community.
I've upped my standards; now, up yours.

Originally Posted by
coyote-1
A soul, waiting to go to God, wanted a turkey sandwich.