RAID in SBS 4.5

I have a Server with Win SBS 4.5 running as our PDC, Exchange Server, Proxy Server, and File Server. We also run an Informix database on it but that is going to change soon. It’s a 233Mhz with 1BG Ram and only 1 6GB HDD. I want to add a 15GB HDD with Raid 1 setup for redundancy. I would like to add a Raid Controller but to save on costs I plan to attach the new drive to the internal IDE controller. Once the new drive is recognized and the drive letters are assigned I can setup the mirror. Is there anything I’m missing??