Everybody in the Mac universe (which is expanding) says to get a second HDD for "output", to keep OS and APP HDD hits from causing dropouts or some such thing. Sounds like very good advice.
Since I use an iMac, can I use a Firewire drive? Or does that all have to go through the processor, making it possibly a worse idea than just using a single drive?

Thanks,

lastmachine

A firewire drive is fine as a second drive.

Make sure you get one with a second generation Oxford 911 chip.

Yep that would be a good idea... Keeping both OS+apps and audiofiles on the same drive will definitely end up causing serious fragmentation problems, thus slowing down your machine.
Firewire is fine... I recently bought one and though I don't use it for recording/playback (I still have plenty available space on my two IDE drives), I've made various tests and found the drive to be very reliable.
As FormatJ said, just make sure it has the "magic" chip (they almost all do now) and initialize it "from scratch" (erase it from the Mac's Special menu in order to remove any dedicated driver delivered along with the drive->otherwise you may run into troubles depending on what audio soft you're using).
Hope that helped.