Theoretically, no existing version of AmigaOS can use more than 4GB of address space, since all pointers are 32-bits wide. Not all of that address space can be RAM either, as you have to map hardware into it too. IIRC, ~2GB is the limit for RAM.
Correction : Some pointers are only 31 bits wide (they are signed and negative values are used as error codes). So the address-space is 2GB, which also has to include onboard components and addons.
