I will put my bed on WD, longer warranty period as well.
Get the Carviar Blue.

If you need to add them into a RAID1 setup then I don't think there's any need to empty other (current) drives.
If you going for RAID0, RAID5, or RAID6 then you have to reconfigure the whole RAID all over means you have to empty all drives, also means you have to prepare another set of hard disk just to safe keep your data before you going for new RAID configuration.
Usually most people going for RAID will purchase the required hard disks in one go to prevent such things happen when you're out of space in current setup.
If you can wait, you should get enough budget to setup another RAID, means of purchase another system with all the required hard disk.
If you can't wait, backup all the data into other hard disk be it external or another PC with enough space, then reconfigure the RAID with your new hard disk then copy back the data.

Good luck on your RAID setup.