Hey rhian,
Interesting questions. Glad you like the site.
A glossary of words is coming, well organized and exhaustive, full of definitions. It is not ready yet but will be here shortly.
I am not a Mac guy at all, but I assume a Mac G4 cube would be better than a G3. As for sequencing, I always push ppeople to get:
.
Cubase VST 5
. VST synthesizers:
Pro-52 (quite decent virtual synthesizer), LM-4 (excellent virtual drum module)
. Non VST synth: Gigasampler or Gigastudio (decent virtual sampler, reading AKAI CDROM even)
You can control all of these synthesizers and samplers within Cubase VST, on MIDI tracks. It's music, so you need to know a bit about notes, but its really fun and pro.
If you are a keyboard guy, get a controller (200 USD or so) which is a keyboard. It will not make any sound but will be helpful to record live what you play (controlling the selected track, which can be a VST synth, the drum module, gigasampler, or anything else a track is assigned to).
Hope this help. I am thinking more and more about making an introduction to Cubase VST as a tutorial (MIDI connections, first steps..). It takes times so stay tuned