1. Get a copy of Norton's Ghost, Power Quest's Drive Image Pro or a similar disk imaging utility. Some vendors will let you download an evaluation copy.
2. Make a backup of your current boot disk. Either place the backup on your current slavedrive or a network drive so you can get at it later.
3. Install the new scsi disks. Set the address jumpers to unused addresses. Usually the boot disk is set to 0 and the scsi controller is set to 7. Most scsi controllers let you boot off of other addresses but 0 is least confusing. Attach the drives to the cable in any sequence. Make sure the last drive on the cable has a terminator and that none of the other drives are terminated. The scsi card should be at one end of the cable and have its terminator turned to "on" or "auto".
4. Run the scsi config program when you boot the system. Most controllers have one. Make sure the scsi controller sees all the drives.
5. Using the drive cloning utility, restore the backup you made to the new disk. Microsoft suggests setting up a small boot partition. This is a good idea. Set the size of the boot partition to 3-4 GB. (You might not be able to make it smaller than the size the clone was made from).
6. Use the windows disk manager to create additional volumes and mirror sets. I don't think you can use "soft" mirroring on your boot partition. You can always use the disk cloning utility to clone it onto the second new disk and set your scsi controller to boot from it in an emergency.
7. Copy your data from the old second (8GB) disk to the new drives.
Notice that throughout this procedure we never touched the contents of your old boot disk. If something goes wrong you can always use it to boot your system and start again. Once you're *very confident* that your system is stable you can reformat and reuse the old boot disk. Personally, I wouldn't touch it though.