Giving up on the PC could be the best thing that ever happened to you.
Cool programs for the mac include but are not limited -BIAB, Reason, Amplitude, Digital Performer, itunes and imovie.
I've been using Garage Band for Mac for the last month and have been having good luck with creating and comosing songs.
There are thousands of real and MIDI instrument loops to choose from if you buy a Jam Pack in addition to the hundreds of loop available with the stock version.
It's $49 and is a part of a bundle called iLife from Apple.
I recommend it if you're running OSX Panther operating system.