... cue now and I got my shiney Mac. I jumped straight in to Logic Express. Do play around with Garage Band, as the workflow is similar (although massively cut down and you get far far better intruments in Logic) but Logic is great, I heartely recommend it - go and play with it in a music shop! I was lucky that a place opposite where I work had an Apple rep just as I went to ask about it and he demo'ed it for me for about 40 minutes. I have never pulled out my credit card so fast in all my life...
I have used a crap load of sequencers in the past (those I have listed, plus Reason and Cakewalk in the old days and until I got rid of my PC recently, I also had Fruity Loops). I'd say Logic is probably the sweetest sequencer I have used so far. There are a few things you need to get used to, but real pro stuff like being able to drag a midi track to an audio track and have it render (instantly) the entire audio track with all effects and recorded event, is something you don't get at this price in Cubase.
It doesn't sound like you need Logic 'Pro'. There are a few differences, like a few less plugins and one of the sampler intruments has slightly more restrictions when it comes to editing waveforms, but that's it. You get a funky FM synth, a couple of analogue synths and an amazing Synth+Sampler that can sound incredible. You also get a load of plugins (the popular ones and a few suprises) and of course you can add your own plugins and synths.
Phew.
Now the intruments. As I said, you get 5 I think, in Logic and they're a great place to start, in fact at least 2 I would say are comparable feature wise with $200 standalone intruments and sound exceptionally good. You can add other Audio compatible intruments. I really want the Yamaha CS80, but it's as expensive as Logic, so I am not going to buy it until I create something that really needs it.
Finally hardware. In Windows I had some meaty setups, but you always got the odd stutter, latency and dropouts, especially when doing things on the screen. This doesn't happen in Logic, ever, unless you deliberately overload the system and start playing Quake. it uses the CoreDuo too, you get 2 CPU bars and it spreads the load (although it's possible for one meaty intrument to take up a whole core if you add enough effects). I use a modest MacBook with a gig of RAM running at 2.0ghz and I get latency on the internal audio of around 40 msecs. Honest! If someone told you they got that on a Dell, you'd laugh and stick them on your ignore list.
Logic! Did I sell it to you?
Not to be too biased, consider Ableton Live 6 (download the demo). It's more for mixing, has amazing drag and drop loop capabilities but also a fairly solid sequencer. Reason is a good intrument base, but I always thought the sequencer was poor. A good rewire app. As for Cubase... for me, overweight and over priced, but you may think differently.
email me if you want - I'll answer any question on Logic.
For $300, it's an utter bargain.