It doesn't really matter if you use an internal drive or an external drive, although an external USB2 connection will be slower than an internal SATA or PATA connection. A program like Ghost is going to see them all the same. If your RAID is done via hardware it should see your two-drive RAID as one single drive.
The reason to have the service tag number is that with it, you can go to the Dell web site and see exactly what hardware was shipped with your Dell when you bought it.
The reason to have the service tag number is that with it, you can go to the Dell web site and see exactly what hardware was shipped with your Dell when you bought it.