Photoshop Build, Storage Question

I am currently in the design process for a system that will be dedicated to Photoshop/Illustrator/InDesign for my brother. I am not familiar with any of these programs and have been doing some research on their requirement. Right now I am trying to decide what I need from a storage standpoint. From my research I have found that the target and source files are best put on different drives. Then the scratch file is best left off the drive with the OS and where Photoshop is installed. I would also assume it is best to put the scratch file on a drive that does not contain the source or target.

So my question is, what is the best arrangement of drives that will be optimized for Photoshop. I have considered the following setups.

A) Drives 1 and 2 in a Raid 0 setup. This will be my C: drive and will contain OS, programs, target and source files for photoshop. Then the 3rd drive would backup drives 1 and 2 and also contain my scratch file. My concern with this setup is all files are on the same drive basically, but they are on a Raid 0 setup so hopefully that would overcome that problem.

B) Drive 1 would be normal C: drive (OS, programs, target) Drive 2 would contain source files and Drive 3 would be where the scratch file is and system backup. With this setup I am concerned Drive 2 would have a lot of wasted space since it is only used for source files.

This is kind of what I am thinking, but I am extremely open to any better ideas or thoughts. I would rather not use SSDs right now because I am not familiar with them and their cost.

I also have a quick Photoshop question about the source and target files. Are the source files basically images you would be importing into what you are working on. Then the target file would be the finished product or what you are currently working on. Would the target file have to be on the same drive as the Photoshop program?

thanks,