Single Port SCSI Card vs Dual Port SCSI Card

  I am trying to make a decision about which card to purchase for a server and
don't really understand the distinction between single port and dual port.
I need to be able to RAID 1 the two OS drives and RAID 5 the four data
drives.  Will I need the dual port for that?  The single port specs say that
it can handle six drives but I don't know if it will allow two different
RAID types.  Considering the price difference, I would like to go with the
single port if I am assured that it will work the way I need it to.

The server model is the HP ML370 and the cards I need to choose from are the
641 and 6402.

Thanks in advance for any help.