A little off topic since you have a mac mini but....
I've tried garageband, logic express 7 and pro 8, pro tools, and others, and from what I can tell the most efficient on PPC is reaper. It takes full advantage of dual cpus because it uses SMP (symetric multi processing).... and heck you can add twice as much tracks and effects vs logic pro 8. If you require CPU power and have dual cpus go for this one!!
Its also the last commercial DAW still supporting PPC macs on the current version. (If not its one of the very very few)
Get latest version with super long trial on official site