The Arcus was the entry level instrument into the CD Range (Spectra etc.) so that any info on the CD line will mostly apply to your instrument as well.
The Mega Tower offered and additional 4 ROM banks (Pre-set Sounds/Style/Pre-sets) and 4 Ram Banks in which you could store your own Sounds/Styles/Pre-sets (The additional banks were in addition to any Memory Card you have inserted)
Later a Giga Tower was introduced which offered even more storage.
Here are some contacts that will be familiar with your instrument.
Hope this helps