If this were my machine, here's what I'd do:
Primary Master: 80GB Maxtor
Primary Slave: DVD-ROM
Secondary Master: Plextor 16x10x40
Then, if you have a RAID controller, I'd run the two 20GB drives in a RAID 1 configuration. I have a few reasons for this. First, I'm deathly scared of running RAID 0. I've lost too much data to even be concerned with the performance gains that can be realised with RAID 0. If you use RAID 1, you will have only 20GB of space, but you will have data redundancy in case something goes awry. My other reason for suggesting this is so you can get the 20GB drive off of the CD-RW's IDE channel, thus eliminating the possibility of too much traffic on the channel during a burn.
I'd partition out the 80 GB drive, putting all of the system files and partitions on a single partition. Then, I'd make a smaller partition to keep the data you work day-to-day on. I'd use the 20GB RAID 1 array to store my more important data that I might have. In my case, it'd be CD iso images and system backups. In your case, who knows what might go on there?