Hi,
You do not need a stacking module for the OS-6800, the module is on-board. The expansion slot on the 48 port chassis is for the 10GIG module. For the stacking you only need the specific stacking cables. The limitation for the 6800 series is that you can't mix the stack with a fiber chassis like the OS-6800-U24 (due to the lack of stacking ports on that chassis type), this limitation is gone with the 6850.
I wrote this just as clarification for the stacking ports - I am pretty sure that GuvnorIBrokeMyPCX already answered your question.
P.S. The last four copper ports are always GBit (as they are combo ports shared with the SFP ports) even on a OS6800-48L(ite) chassis, so you can gain some extra bandwidth for your "uplink connection" between the two chassis.
Regards,
Benny