[G4] Additional HDs and Photoshop

[Robert C. Buitron]

I have a G4 mirrored doors drive, 1.25 GHz, dual processor/ 80 GB ultra 
ATA HD. It has max RAM - 2GB. I have 2 Maxtor MaxLine II ATA/133 IDE 
300GB hard drives and an ACard  ATA133 controller/adapter PCI card 
(AEC-6280M) that can support up to 4 HDs (Master/Slave).

Can I partition the original 80GB HD so that one partition holds the OS 
and apps and the other partition serves as the scratch disk for 
Photoshop? The main reason for the two 300GB HDs is storage. I've nearly 
max-ed on the existing HD and I'd rather have image files on HDs rather 
than on CDs or DVDs (takes too long and too many disks to transfer 
stuff). I also would like to use the two HDs as masters in order to 
maximize bandwidth speed when retrieving and storing files. I do have a 
portable 40GB HD, however I use it for transporting files as well as for 
some backup storage. I read somewhere that Photoshop prefers an entire 
disk for its scratch disk. It seems a 300GB scratch disk is way too 
much. My image files range from the KBs to the GBs. I mostly have files 
on the larger size (especially the working files with all the layers and 
stuff). Any recommendations (and I don't have RAID capability or more 
money for external HDs), answer to this strategy, or better way to get 
some increase in rendering/processing in Photoshop with the above 
hardware and very large files are welcomed.

I'm not very tech savvy about this, just enough to get by, so please 
indulge me and give a clear, step-by-step answer or strategy of how to 
set the hard drives up for what I have in mind.

Thanks,
Robert