Another Darn Beginner...., Recent convert to Mac requests help! Options

[timski]

I bought a Mac last year and have just decided to get back into music recording. I used to use Cubasis with the E-Mu APS and Wavelab on my PC (going back a few years this is) but would like advice on the simplest, best value and least bulky set-up. I am not hoping to record very complex, amazing sounding tunes, just be able to put my ideas down as smoothly as possible. Roughly 6-10 channels of audio and 4-6 channels of MIDI.

I have a Powerbook G4 (40GB HD, 256MB RAM) which has the latest Firewire and 2xUSB2.0. Most of my recording will be audio - guitars and basses, vocals, analog keyboards (Rhodes Stage Piano, Wurli 200, Hammond B200), a variety of other real instruments (mandolin, violin, accordion, hand percussion) and possibly sampling some of my old vinyl but I will need MIDI to access other sounds, drum beats, etc. I also have a Studiologic SL-161 master keyboard (MIDI connections only).

My main questions are:
What's the best way to connect everything up? Can I easily connect the master keyboard into a USB port? If so, what's a good device to use?
Should I use a separate input for the audio interface or can I get away with using the audio-in port on my Powerbook?
I currently just have garageband (so cheap I couldn't resist!) Could I just use this or should I really get some 'proper' recording software?
Is it OK to run all of this alongside all the other apps I frequently use or do I really need to have a computer dedicated to music only? And how much HD space and RAM should I leave free?

It's been at least three years since I really did much recording and things seemed to have moved on a huge amount since then.

Any help would be greatly appreciated!!
Thanks