Hard Disk Space Needed For Audio?, planning my setup Options

[keybie]

I'm trying to determine how large an external Firewire harddrive I need music/video use.
I assume this is a very general question, so let me be a little more specific.

First of all, I haven't even bought the Mac yet, so I have no idea what kind of software I'll end up using. I have however decided on buying the newly introduced 15" Powerbook G4.
As for "making music". I've got lots of external hardware MIDI modules, synthesizer keyboards etc. which I currently control from my trusty Atari Mega STe running Notator SL sequencing/score software. Since I'm happy with this solution I want to continue using it that way, while the Mac will become a "virtual recording studio", syncing to the Atari computer via MIDI and recording the audio.
I have a couple of effects-processors, but adding effects within the Mac would probably be a lot more practical. A software sampler would also be very welcomed as my hardware sampler isn't very user-friendly with its floppy disks etc

Now, I don't know what audio software package would be most suitable for my use, but it looks like Emagic/Apple's "Logic express" is one of the most mentioned ones around.

Anyway, given the above information, how much disk space do musical projects in general take up? I'll have to add this to my video editing of course, but that's another thread/forum of course