How you do it kind of depends on which kind of RAID you will be running, Software or Hardware. I did it using software RAID and followed these steps:
1) I installed two Promise IDE cards (non-raid) into the system. I connected 2 40GB IDE drives, one as the Primary Master on the motherboard and one as the Secondary Master on the motherboard. These drives will be the root filesystem mirror. I connected a DVD-ROM as the Primary Slave and connected power to all drives.
2) I connected 4 80GB drives, each one as the Master on the Primary and Secondary channels of each card. I did not connect any slave drives. This ensures that the drives operate at full throughput. Now the important bit. DO NOT CONNECT THE POWER TO THE DRIVES YOU WANT TO USE IN YOU RAID 5 ARRAY UNTIL THE SME INSTALLATION IS COMPLETE.
3) I installed SME and chose the mirror configuration. SME detected the promise cards.
4) Once SME was installed, I shut the machine down and connected the power plugs to the RAID5 drives.
5) I started the machine and added the RAID5 config to /etc/raidtab and started the raid. I formatted it with an EXT3 filesystem and mounted it on /mnt/test. I then copied the contents of /home to /mnt/test. I then unmounted /mnt/test, did an 'rm -fR' in /home, and mounted the raid on /home.
6) I edited /etc/fstab to mount the raid on /home and rebooted.
I then had the root filesystem on a mirror and the /home filesystem on a RAID5. Sweet!