Advise for my new I Mac, I need Midi and audio help Options

[mtrev]

I have recently bought a new I Mac 700 MHz, and am anxious to make some music with it...I need some advise.

I want a program to compose and arrange music...I want to use my synthesizer via midi, as well as use outside audio like voice, guitar, sound loops etc...
I want to be able to manipulate sounds, cut and drag, and notate.  I also want the capability to receive audio from my mini disc player.  

I have been researching Cubase and think it would be perfect.  Any thoughts on this program, and whether it might be right for me?  Which version?  

Would I need to by a sound card as well as the software?

I know I have a lot of questions, but ANY advice and/or wisdom would be much appreciated...