I've been a Cubase user for years (since the Atari!) and have obviously got VERY comfortable with it, to the point where I hate most other things (especially Pro-Tools!) for MIDI use...
BUT.... I have to explain my situation. Things have changed since I got my rig. I still use an ancient Quicksilver 733, and still do most of my MIDI and audio work in OS9, and use Cubase VST5. Primarily because everything works close to perfectly, including my UAD-1 cards, my PCI MOTU audio interface and my Midex3 MIDI interface, all of which have issues in one way or another with OSX versions of Cubase, or newer Macs. I come from the 'if it works, don't change it until you HAVE to' school of gear (unlike Donny
) and for all my work outside the big studio I work at, my rig works fine....
However, on newer, Intel Macs, you have more choice (and power!) than I do. To be honest, I would STILL recommend Cubase for MIDI work, as the basic look and feel hasn't changed too much since VST5, but as a newcomer, I would also possibly suggest you take a careful look at Logic. On the audio side of things, this is perhaps a bit more efficient than Cubase right now, but it's MIDI editing I still find a little clunky, but that is quite possibly due to being SO familiar with the 'Cubase Way'.
So, if you left it completely up to me, I'd say a mid-line version of Cubase, maybe Cubase Studio 4. Any MIDI interface (except Steinberg's own Midex interfaces!) should do the job. I would spring for a multi-port interface if you have (or expect to have) more than just one keyboard or module. I'm sorry I can't help more with OSX specifics, but a trip to Steinberg's own website might find you more timely info.
Lastly, do a bit of research on Apple Logic Studio. For the price (around $500), it has an unmatched set of VSTi's, VST plug-ins, and capability. I may very well try it out when I finally break down and get a new Mac Pro...
Hope this helps (a little!)