I've been playing with GarageBand, but even after installing 1GB in my iBook, have been a little annoyed with it's performance. I mean, I think GarageBand is perfectly fine for me features wise; it's just annoying to have it stop recording after only three or four minutes with only one track.
Apple also sells Logic, and Logic Express. Logic is a high-powered audio creation and editing tool, while Logic Express is somewhat in between GarageBand and Logic. Some people on the web report that Logic is more efficient - less of a system pig - than GarageBand.
You can even download a trial version of Logic Express, though I found the link on another web site rather than found it on Apple's site, itself.
So I'm going to give that a whirl and see if it really is a little smoother than GarageBand.