the professional one i think is logic pro 7 but its mega expensive, but you could def download it on some torrent site somewhere. then there are millions of small ones....mac world and those magazines always use to feature them....so maybe check that out?
garage band is fun for like, an hour. i dont have a mac but garage band is limiting. personally i like reason and im pretty sure it just came out for MAC's recently. it can also hook up keyboards. it has a pretty simple and clean interface so thats good too.
i'd try reason...programs like cubase, sonar, and logic are sequencers so you can't really make beats with the program alone...you'll need VST's which is fine cause there are some sick one's out there like Kontakt and Battery...with reason you can make a bea start to finisht with just reason and it is pretty easy to learn and has so much that you won't ever find yourself limited