Piru is right, the A3/4K fast slot has a ram chip select generated by the motherboard, this chip select is for a 128meg window, you can not add more than that without adding a full on autoconfig system to the board.
The A1200 doesnt have a ram chip select on its trapdoor slot so they have to autoconfig it. (they have the Autoconfig signals so its just like a zorro slot)
Zorro 3 has autoconfig so it can specify larger sizes or let the OS size it so thats fine.
You can not add more than 128 meg of ram to any fast slot CPU card that uses the ram chip select generated by the board and I know of no card that autoconfigs the ram.
So tadaaa.
if you want more ram make a Z3 board.. and Piru knows what he is talking about. :-)
edit:
GVP Trex040: Max Ram: Unknown but probably 64MB without RAM board and 256MB with RAM board. This is wrong then?
Wasnt the biggest 64pin simm only 16meg? cause that would make 128meg max ram and tadaa.
Oh and just checked, Amiga Resource
says its for 4 or 16meg simms