Hard Drives on a Studio 1749

Hello all, I have a rather dumb question, one which I should have got answered BEFORE I ordered rather than now, it is more or less a question of just curiosity.

I just recently ordered (and subsequently received) a new Studio 1749. I ordered it with dual 320gb hard drives. And I can see in my device manager I have two Toshiba mk3256gsy hard drives. My question is, is this two totally seperate hardrive units or is it just one box (device) that has two platters inside it? (which if it is sorta defeats my reasoning for wanting two harddrives). And if they are two seperate units, then could I (if I chose) at some point in the future  go ahead and replace just one of them if I decided I need more storage? (right now I have the OS and  all programs on C: and all my data and stiuff on the other)

I am also curious  as I was sure on dells website that these were listed as dual sata drives, but my device manager doesn't show any SATA controllers installed, and lists these drives as ATA5 on an IDE controller.

Just curious to know, exactly what do I have.