Amiga RAM addressed through the motherboard is limited to 8 MB, the higher addresses are used by the system to access chips and such. The PCMCIA slot, being on the motherboard "uses" the lower "addresses" that would be used as RAM, so that anything taking more than 4 MB overlaps those addresses.
On the other hand a add-in CPU board WITH it's own RAM addresses that RAM over the CPU-board's address lines and being separate from the motherboard, does not interfere. The maximum "buy-able" CPU card with RAM is 256 MB; the maximum usable RAM is 2 GB by Amiga OS; you will find this on AmigaNG motherboards. The Maximum RAM on a Classic Amiga is a Amigakit demo of 1 GB of RAM; this is in the Z3 address range and precludes use of adds-in boards on the Z3 bus making the machine very limited.
I appreciate clarification if some of this information is incorrect
