Question re: User Guide instructions on adding a second hard drive.

I recently lost a WD 320 GB HDD on my Dell Dimension XPS GEN5 system. I am in the process of replacing that drive as well as adding a second HDD. I was reading the User Manual and found some information that I do not understand. Specifically:

5 Remove the first hard drive from the upper bay and install it in the lower bay:
a Disconnect the power and the hard-drive cables from the back of the first hard drive.
b Press in the two rail tabs and pull the first hard drive out of the upper bay.
c Gently slide the first hard drive into the lower bay until it clicks into place.
d Reconnect the power and hard-drive cables to the back of the first hard drive.
6 Gently slide the new hard drive into the upper bay until it clicks into place.

It looks to me like the HDD cage (bays) is just a physical container to hold the drives. Why would I need to relocate any hard drive when replacing/adding another hard drive? It seems to me that what is important is that the SATA cables be connected appropriately. Which leads to the question: Does it matter which SATA 0-3 cable is attached to which drive? Does the boot drive have to be connected via the SATA 0 cable/port?

Last question: Since there are 4 SATA ports (0-3) and the specifications state that this system only supports 3 HDDs is it OK/possible to attach an external eSATA drive from the fourth SATA port?

I would appreciate any thoughts/comments anyone might make.

Thanks,

Keith