Logic is a very good app to run under single processor machines, I run it occasionally on the PowerBook, but we run it all the time on G4's at the Uni, and some of those are only 500Mhz...
You will be able to run many more audio and midi tracks in Logic Express, up to a limit of 250-odd for audio, but you'll only be able to record 12 tracks at once. Garageband is a v1.0 freebie that allows you to play around with audio with no real knowledge of music, sure it has some powerful features, but it isn't really an iMovie analogue, iMovie is quite capable of turning out decent results, GB really needs to horsepower to be remotely useable.
This is true of ALL audio workstation apps, if you run them "native" they'll suck your proccesor dry, they really do need the off-board power of dedicated DSP cards like the Digidesign Accel cards, or an external interface like M-audio's 410.
My ProTools LE will only run to 35ish tracks and a few plug-ins before running out of legs, there is no substitute for DP power with audio.
Logic will run like a charm and is a MUCH more creative app than Garageband.