Goddamn - That's a hot setup, dubdub. Wish I had something like that... but my 800MHz iMac Widescreen seems to do the job just fine.
Assuming your hardware checks out fine, there are a couple of things you might what to check: CPU drain, system preferences and (as an outside contender) disk fragmentation. Within GarageBand, use MIDI loops wherever you can.
CPU Drain
Are you running background activities on your powerbook that might be stealing CPU time from GarageBand? Common activities might be SETI@HOME-like screensavers which you're running in the background, running Classic and Classic applications while running Panther, other applications that might be draining CPU power while running, and (heaven forbid!) worms or viruses trying to melt your system like a blowtorch to candy-floss.
If you're running Panther, checkout a handy Utility (in your Applications > Utilities folder) called "Activity Monitor". I won't explain its function - hopefully it's fairly straight forward - but you should be able to see if any programs (the 'Process Name' column) is gobbling up your CPU usage.
Activity Monitor is a handy-dandy tool to examine various eldritch, arcane processes that make computers go well... and some processes which might be spawn of the devil.
System Preferences
(Ay Carumba... I so don't want to go here...)
Welp... The big drainers here are screensavers, over-active virus detection agents, and possible hardware add-ons that demand RAM. This is not really a significant area, but it can be a highly sensitive one, because different people need to run different extensions, depending on the primary function of their comp. Just... Just turn off the eye-candy wherever you can
Disk Fragmentation
I don't think this is the problem area either, as you say you have a new comp... But it COULD be a problem area if, as soon as you got your comp, you loaded all sorts of OS X and Classic apps in random order, worked on a few files, deleted some, updated others and installed GarageBand a couple of times, in between working on a few Photoshop files bigger than The Beatles.
It could be that your GB file has been edited and saved so many different times that the file has fragmented, that is, it has been stored in several locations on your drive. When this happens, your drive has to do a lot of seaching to put the thing together and load into RAM.
Make no mistake - This one is a LONG shot. I'm having a lot of trouble convincing myself that this is a problem worth considering. But it may well be a problem in the future. Consider getting Norton SystemWorks or TechTool Pro, something of that nature, to ensure your disk doesn't become paralytically slow.
If these three areas aren't the culprits, I'd recommend re-reading the GarageBand spec sheet, compare it with what you have, and drag your comp back into the shop you bought it from and say "Hey! The Hell??!?!...." and get some advice. Your comp might actually have a defect in it. Just because new stuff is new, doesn't mean it's flawless.
MIDI Files
(I'm afraid this doesn't solve your problem. It just helps you run GB economically, as far as processing power is concerned.)
GarageBand handles three different types of files: Apple Loops, original recorded input (which includes waveform files you might import or drag/drop in to the application), and MIDI files.
MIDI files use far less processing power and RAM than the other ones. If you can, select Apple MIDI instrument files over waveform loops to serve as your track data, or use an electronic input MIDI device (keyboard) to create MIDI instrument files.
Hope this helps,
-Oro
