|You will not want to change position of the existing drives in the cable. Yes get a new cable.|
Make sure you back up everything as there is a risk factor here.
Personally I never partition OS and data on a single raid5. SOP is to mirror the OS and raid 5 the data.
The reason for this standard operating procedure is just for what you are going to find yourself in shortly. It is rarely successful to add space to both the OS partition and the data partition. It is far better to wipe the data partition, add the drives, expand the OS partition into the raw area and then recreate the data partition and restore from backup.
You also get better performance if using a two channel card with OS on one channel and data on another.
The problem these partition managers have is moving the data so it can add space to the OS partition.
Its like you adding a bathroom in the center of the house while moving the rest of the house out of the way to provide the space for the bathroom.
I also highly recommend you add another drive as hot spare. If you have two drives fail you have lost everything. A hot spare should give you a margin of error in case of drive failure.