i use garage band 2 on my 15in 1ghz powerbook and it does okay. course i also have a gig and a half of ram. so that helps, alot. But when it only had 768megs it was struggled. so yeah, i'll just repeat what everyone else says just to think i'm part of the cool club.
2.4gHz 15" MacBook Pro, 1.66gHz Core Duo Mac Mini, 2.5gHz G5 QUAD, 733mHz Quicksilver, 450mHz G4 Cube, 700mHz G3 iBook, 350mHz Sawtooth G4, 350mHz Revs. A and B B&W G3, 16mHz Powerbook 100, 8mHz Macitosh Classic.