A scratch disk that's separate from your "C" drive will provide improved performance for Photoshop when doing large file reads or writes of information, because it allows the computer to access each drive at the same time and independantly of each other. So one drive is paying attention to the Windows instructions and application operation ... while the second drive only pays attention to the data it's reading or writing for your image. (I'm oversimplifying - but that demonstrates the concept.) When everything's run from the same drive, your computer is constantly moving its drive heads on your disk back-n-forth really fast to do everything at once. It may be reading a Windows instruction for a few milliseconds or running a background operation that requires disk data, then jumping to the location it's now writing that 40MB+ file you just created ... now back again for Windows ... then back to write file data ... etc. Although we're talking very short periods of time for the head to switch to different sectors on the disk and jump to and fro - it adds up when working with large files.
I don't have any performance numbers to toss at you, but you'll notice a substantial improvement when working with LARGE files. Not so much on little stuff. But be sure to buy the fastest addt'l drive you can such as 7500-10,000RPM and don't waste your time with 5,000RPM drives or USB attached disks. In other words: if it isn't using your drive cables INSIDE the computer and your built in bus speeds - you won't get noticeable improvements with a scratch disk in most cases.