Here's what I've done so far:

1. Removed the controller card.
2. Swapped out the ASUS CD-ROM for the Plextor CD-R (which was on the controller card)

I'm now running my configuration like this:

Primary Master: 10GB IBM
Primary Slave: 40GB Maxtor
Secondary Master: 80GB Maxtor
Secondary Slave: Plextor CD-R

I'm going to see if this stays stable. I'm thinking I'm going to get one of those WD 8MB cache drives (either 80GB or 120GB) and swap that in as the Primary Master with at least two partitions. Then I can get rid of the 10GB and 40GB, allowing me to swap my CD-ROM back in and not need a controller card.