PERC4ei (or PERC4e/Di) is the RAID on motherboard (ROMB) version located on the PCI-Express bus on the riser card.  It is used to control internal drives.

 

The PERC4/DC has a PCI-X bus attachment, and is an add-in card using a slot on the riser.  It can support either internal or external drives.  The /DC is the PERC4 card that supports clustered applications of the PV220S SCSI disk array.

 

If you are only using internal drives the PERC4ei may be the way to go.