I'm new to this too.
I recently purchased a G4 to use for my graphic design work
as well as home recording. Many people talk about soundcards ... but
I just burned a CD using a RCA to mic input from my synth
and I was pleasantly suprised of the quality of the recording.
If you just want to do quality demo recordings ... then (I think) all you need
for now is a digital audio recording software (like
Emagic Logic Silver
$250 or so) ... and a way to imput the signal to the mic in the back of
the computer. The software I mentioned has some mixing capabilities.
Again ... I'm just recording electronic instrumental music
from a synth. It may be a different story if you are recording accoustic ...
but I thought I would try and respond since nobody else is.