Well I would start off by looking at the application and database requirements. IE How big will the database be. Is it read only or read write, How many users, How many simultaneous users, what...
You must do the metaroot command and then reboot the machine before attaching the other half of the mirror to the root device. In most cases people forget the reboot which causes the below...
Like Seg said raid manager is for hardware raid. I would use ZFS, which is built in with Solaris or Veritas Volume Manager an add on. Veritas was the preferred method but now with ZFS being ready...