Apple Logic Pro /LogicExpress Discussion

I just purchased a 500GB Maxtor drive, 800Mb FireWire.  My system is a new iMac, 2GHz cpu, 
2GB memory, 320GB drive.  Since I've loaded on the software and some video footage, 
already I'm 'down' to 200GB free.  

So I bought this external drive and realize I'm unclear exactly how I want to use it.  There's 
several large video and music projects in the near future.

I hear Logic works better when the audio files are on a fast external drive.  I'm also tempted 
to simply import new video footage onto the external drive.  But this scenario simply expands 
my available space and does nothing to protect my data.

I could partition the drive, part as backup, part as overflow.  But that gets nonsensical.. if 
part of it's a backup area, using SuperDuper or CCC or even (gasp) Time Machine, and then I 
use the other partition for audio or video storage, all I'm doing is NOT backing up that stuff 
and also betting that the external drive will last longer before failure than the system drive!

So.. help me think my way out of this paper bag.

thanks!