[G4] 2nd Hard drove for G4 MDD

[Carol]

I have a G4 (Mirrored Drive Doors) with the original 160 GB hard drive. 
  I am thinking of getting a 2nd hard drive.  The manual says that this 
model can have up to 4 ATA or Ultra ATA hard drives.  It further states 
that Ultra ATA/100 drives should be placed in the rear bay only.

What is the difference between ATA/100 and ATA/66 drives?

I was told by an Apple service provider that one can boot only from hard 
drives placed in the rear bay drives. He also said that the new G5s use 
different kinds of hard drives. Can anyone verify this?

My plan would be to get a large Ultra ATA/100 drive, say 300 GB or 
larger and partition it so that one partition was the same size as the 
boot drive.

I would reserve this partition for backups of the boot drive. Should I 
be able to boot up from the backup and use Disk Utility on the source 
hard drive or would it be better to load a basic version of OS X on a 
2nd partion for this purpose?

--
Carol