Its all about the rpm's of your hard drive. Here we go:
A SCSI drive is RECOMMENDED but not a must. A SCSI 10,000 rpm drive would be ideal. This would allow you to run above 18 tracks, and have no slowing down issues on the graphics. (like the faders not keeping up w/ the music). AUDIO IS NOT AFFECTED.
If you are talking about the 001 system, then i would recommend AT LEAST a 7,200 rpm drive. The processor power isnt as crucial for PLAYING BACK audio. Dont get me wrong, the more RAM/Processor you haver the better, But the drives speed is MOST important. If you plan on recording live with more than 18 tracks, then get a 10,000 rpm drive. If you only want to use it for recording a couple of tracks at the same time, then a 7,200 rpm drive is fine. UNLESS you plan on playing back more than 18 tracks at a time.
I use a Western Digital 7,200 rpm. Because they didnt offer the SCSI drives when i got my G4. Anything above 18 tracks, and the faders dont keep up with the audio. THE AUDIO IS NOT AFFECTED, just the visuals of the faders during playback.
The 001 system comes with a pci card. The MIX PLUS systems, come with "farm" pci cards. These cards use their own built-in processors, and the quality is unbeatable. Thats is why those sytems start at about $5,000.......
I have been using the 001 since about 2/2/00, and Im sure i can answer any questions you might have. I have ownded my own recording studio for about 3 years. I have recorded my own bands songs, and numorous other 'local' projects.
Good luck