Yep all the higher end Canon printers use the 6 series cartridges.
The 860 (and new iP4000) Use the CYMB plus a 3 series Black
The i900,i960, and wide carrage i9100use CYMB +PM+PC (same as your current s900)
The i9900 uses the CYMB +PM +PC +a new Red and a new Green.
I also had an S900 and replaced it with a i860 the quality is actually (IMHO) better than the 6 color S900, plus I got the fast and cheap Black printing.