Creating Mirrored RAID on Startup Disk

I have a Mac Pro that I want to create a mirrored RAID set in.
I have a 1TB Hard Drive that is the main system drive.
I just added a 2nd 1TB Hard Drive that I wanted to use along with this first hard drive to create a mirrored RAID set.
I used the following document to help with the process:
http://docs.info.apple.com/article.html?path=ServerAdmin/10.6/en/asa772e2fc.html

I have opened up Disk Utility and think I may know how to do this but a few things concern me.
First, it states that it will erase the contents of the drives it's using to mirror. I had the idea it would simply duplicate the data onto the other drive. Is it true that it will delete the data on the current drive?

Also, it states that you can't create a RAID with the startup disk. Is that true? I currently have a RAID in this system using the disks in bays 3 and 4. I purchased two 1TB disks and created a SERVER DRIVE raid and this drive is shared with all computers on the network as a data drive. I had no issues setting it up but of course both drives were empty when I created this RAID and also these are not the system/startup drives.

Is there a way I can create a RAID with the main Macintosh HD system/startup disk and also do it without erasing the data from the main drive?

Thank you for your help.

 

Thank you for the information you provided. The more I'm looking at it instead of going the RAID route I'm considering Time Machine. I have used that successfully on other systems and it would back up hourly and if I delete a file it won't replicate onto another hard drive. I am sure Time Machine slows down the system some but I think it may be a better option for me to use than RAID. We also have an offsite backup which is done weekly.